In the meanwhile, you can extend the list in notation.class using local.notation.class to allow PDF by adding this to your DOCTYPE in each file:
<!DOCTYPE book PUBLIC "..." "..." [ <!ENTITY % local.notation.class "| PDF"> ]>
That should enable validation.
It seems that xmllint can't handle that extension:
/opt/gnu/bin/xmllint -valid --noout LEDATutorium.xml
LEDATutorium.xml:44: validity error: NOTATION attribute application reference an unknown notation "PDF"
<book lang="de">
^
chapter1.xml:6: validity error: NOTATION attribute application reference an unknown notation "PDF"
<chapter>
^
(using libxml version 20412)
This is a different error message from what I get without the above extension:
/opt/gnu/bin/xmllint -valid --noout LEDATutorium.xml
LEDATutorium.xml:752: validity error: Value "PDF" for attribute format of imagedata is not among the enumerated set
<imagedata align="center" format="PDF" fileref="Pictures/ArrayOfChar.pdf ^
nsgmls reports the file as valid now.