Is your feature request related to a problem? Please describe.
I often download PDFs to read, and only sometimes want to annotate them and then save the result. By default, downloaded PDFs are saved to /tmp/, and deleted occasionally when cleaning up (i.e., on reboots). If I decide to annotate and keep the result, I can export the resulting annotated PDF to ~/Documents/. However, I cannot save the resulting .xopp file to ~/Documents/, as the .xopp refers to /tmp/ to find the PDF, which will at some point be cleaned up

Describe the solution you'd like
I'd like the ability to save "PDF + XOPP" files to a given directory. One option is to have a distinct option for "Change source PDF location", which would update the URL to the PDF, and save the PDF to a new location. Another option would be to save the PDF within the .xopp file as some sort of package.

Describe alternatives you've considered
My current workaround is to manually script "move_pdf_to_xopp.sh", which gunzip's the .xopp, extracts the PDF location, copies the PDF to the current directory, updates the unzipped .xopp, then gzips the .xopp back up.