XReader: add file annotations

[timon]

I miss a feature that allows me adding "attachments" (I believe they're called "File Annotations") to PDF file with XReader.
While XReader displays file annotations added to PDF just fine, I miss possibility adding new files

[Jopp-gh]

Xreader is a very good software but would need some more features with the focus on studying. I miss in xreader the ability to export annotations.

Since Mint's Dev team is small and busy, get Zotero

[exegesisget]

It's a good idea, but I'd also like to add rtf file support to xreader.