Give Zotero some time. My fiancee used it while writing her Phd thesis.
Although she was quite new to free code software at the time, she found it
superior to EndNote, which is what she used before. She paid the very
reasonable annual free that gave her access to the remote storage and sharing
features. If you don't want to help fund ongoing development, that's fine,
AFAIK all the source code required to set up your own remote backup and
sharing server is here:
https://github.com/zotero
I've not tried, but I imagine there's a way to integrate Zotero with a
full-featured, free code PDF reader. Something like Zotfile?
https://github.com/jlegewie/zotfile