I thought it might be good to pool our nerdy knowledge on this one.
The file size for PDFs at Libcom is 32megs.
There's a trade off between file size and legibility. A few of us are self taught contributors to the site - whether we're scanning our own documents or finding them online and adding them here.
All the tools I use are online and free, but some of them limit the size of files you can use, or how many operations you can do in an hour or day:
https://www.ilovepdf.com/
A nice general tool for manipulating PDFs. Compression function is good. You can also merge, split etc. I use the "organise" tool quite a bit for moving pages around, deleting them, etc.
https://smallpdf.com/
Same as above, but much more restrictive in how many operations you can do at a time.
https://www.sejda.com/
This is the best tool I have found for cropping PDFs. Quite useful if you have a document which has been scanned as double pages and you want to change it to single page views.
https://www.onlineocr.net/
This is the best site I have found for Optical Character Recognition - basically extracting the text from a PDF or jpeg or whatever.
If you use other things, let me know!
you can do basically anything
you can do basically anything to PDF files (file format developed by Adobe) if you "obtain" a copy of Adobe Acrobat, including stuff like OCR conversion ("converting image-text to actual text" etc.). not sure how helpful that is, but it's basically all that I use when working with PDFs