During undergrad I amassed a huge library of textbook pdfs, djvus, etc. but once I actually started trying to study from them I realized how frustrating it is trying to read an entire textbook that has soykafty digitization quality; not searchable, no table of contents/equation links, etc. In cases where I can't find higher quality digitizations of books I prefer to buy physical versions (where at least I can use page-markers) now.
This thread is for posting and requesting digital textbooks which are at a minimum searchable, but preferred with working index and links.
If particular textbooks without good digitization are highly requested here I may consider buying a copy and paying for high quality digitization, and if you have ever paid for a high quality digitization we would all be grateful if you would share the result.