Ok, here’s a quick search using google’s advance operators that would allow you to search for pdf books online. Many people upload pdf version of books to sites, and using this advance search you can find them.
Here’s an example search, in which the title of the book is surrounded in quotes, which means search for this exact title. Then you use the “filetype:” operator to tell search engine about the file type you’re searching for, and finally, since many site pay google to show first in searches, like amazon and other books online retailers, you want to eliminate those from the search, for this, you use the “-site:” operator. The minus sign in front of it, means to leave out any sites specified here.
"Programming Challenges: The Programming Contest Training Manual" filetype:pdf -site:amazon.com