You could try Sci-Hub and LibGen. If you don’t find what you’re looking for, you can always email the authors or other scholars who may have access to the paper you’re interested in. A less alternative site would be arXiv, but it just has preprints.

To the best of my knowledge, there isn’t a good FOSS PDF editor.

There’s LibreOffice Draw, but as you said, it messes up the formatting. There’s also Inkscape, but good luck if you have pages of text. You can also try Scribus, but I wouldn’t say it’s good (or intuitive).

If all you want to do is write over a PDF, put (not edit, nor remove) text, formulae, or images, then Xournal++ is very decent.

If I need to edit a PDF, I use (*gags*) Adobe Acrobat running in Wine (with Lutris is very easy). It’s as proprietary and evil as it can be, but it’s good at editing PDFs.