Google Books is an excellent source to get free PDF for many books, and some magazines, from before 1923. (Anything published in 1922 or earlier is in the public domain in the U.S.) Google Books also lets you view, but not print, some pages of more recent books that are still in copyright, and this has created controversy; but I won’t go into that here.
From the Advanced Book Search page, you can limit your search to “Full view” books and magazines, and search by title, author, date, or even a phrase in the book (use the “Exact phrase” box for that).
Many research libraries have made rare books available for scanning, and I have found some unusual items in Google Books. In one case, I found a printed rehearsal script of an 1894 play that included material cut before the opening. I contacted an expert on the playwright, who told me that he had seen this in a museum, but had been unable to get a copy.
Google Books is especially exciting for librarians. In addition to finding answers to research questions, I’ve been able to “fill” several interlibrary loan requests for books from the early 1800’s (and even 1700’s) by providing patrons with links to the books they want. One patron wanted an 1850 book that no library in the US owns; it’s apparently available only at three libraries in the UK … and in Google Books.