I can confirm this bug. Using the current up-to-date Debian Stretch(testing), opening any pdf file with xpdf results in these messages being sent to stderr:
Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-arabic:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cidToUnicode' (/usr/share/xpdf/xpdfrc-chinese-simplified:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-chinese-simplified:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-chinese-simplified:4) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-chinese-simplified:5) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cMapDir' (/usr/share/xpdf/xpdfrc-chinese-simplified:6) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'toUnicodeDir' (/usr/share/xpdf/xpdfrc-chinese-simplified:7) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'fontFileCC' (/usr/share/xpdf/xpdfrc-chinese-simplified:8) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cidToUnicode' (/usr/share/xpdf/xpdfrc-chinese-traditional:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-chinese-traditional:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-chinese-traditional:4) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cMapDir' (/usr/share/xpdf/xpdfrc-chinese-traditional:5) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'toUnicodeDir' (/usr/share/xpdf/xpdfrc-chinese-traditional:6) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'fontFileCC' (/usr/share/xpdf/xpdfrc-chinese-traditional:7) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'nameToUnicode' (/usr/share/xpdf/xpdfrc-cyrillic:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-cyrillic:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'nameToUnicode' (/usr/share/xpdf/xpdfrc-greek:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-greek:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-hebrew:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-hebrew:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cidToUnicode' (/usr/share/xpdf/xpdfrc-japanese:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-japanese:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-japanese:4) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-japanese:5) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cMapDir' (/usr/share/xpdf/xpdfrc-japanese:6) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'toUnicodeDir' (/usr/share/xpdf/xpdfrc-japanese:7) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'fontFileCC' (/usr/share/xpdf/xpdfrc-japanese:8) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFontCC' (/usr/share/xpdf/xpdfrc-japanese:38) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFontCC' (/usr/share/xpdf/xpdfrc-japanese:39) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:40) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:41) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:42) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:43) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:44) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:45) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:46) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:47) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:48) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:49) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:50) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:51) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:52) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:53) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:54) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:55) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:56) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:57) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:58) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:59) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:60) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:61) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:62) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:63) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:64) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:65) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:66) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:67) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:68) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:69) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:70) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:71) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:72) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:73) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:74) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:75) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'psResidentFont16' (/usr/share/xpdf/xpdfrc-japanese:76) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cidToUnicode' (/usr/share/xpdf/xpdfrc-korean:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-korean:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'cMapDir' (/usr/share/xpdf/xpdfrc-korean:4) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'toUnicodeDir' (/usr/share/xpdf/xpdfrc-korean:5) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'fontFileCC' (/usr/share/xpdf/xpdfrc-korean:6) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'fontFileCC' (/usr/share/xpdf/xpdfrc-korean:7) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-latin2:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'nameToUnicode' (/usr/share/xpdf/xpdfrc-thai:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-thai:3) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf Config Error: Unknown config file command 'unicodeMap' (/usr/share/xpdf/xpdfrc-turkish:2) Config Error: Some work needs to be done to suppport this option in the Poppler version of xpdf