Project Gutenberg has a mathematics section, and they prepare their more recent publications in a format that works very well on an ebook reader of moderate size: they generate PDFs in a size of $8.5\mathrm{in} \times 11\mathrm{in}$, with 12pt tex. They have some very interesting books.
I like this very much and therefore would like to know whether there are other websites that provide mathematical texts in this format.
Edit: After seeing the first answers I see that I had not explained clearly enough what I am asking for. I am not so much interested in mathematical PDFs in general, but in those that are optimised for small ebook readers. Most PDFs are made for printing, they consist of A4 pages with wide margins, and to read them comfortably I have to print them or by a larger ebook reader or tablet. The PDFs of Project Gutenberg have small margins, relatively large letters and a page ratio of approximately 5:4 and therefore display very well on a small ebook reader.
I have seen them until now only at Gutenberg, but would like to find other sources too.
Best Answer
I've downloaded a lot for my ebook reader from http://www.e-booksdirectory.com/mathematics.php
That's just the math section of a larger site. Mostly they're links to lecture notes and such whose creators have put them up for free download.