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:
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.
Same as above, but much more restrictive in how many operations you can do at a time.
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.
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!