As gwynevens says PDFs which are only text should be readable. They are best read at 'medium' font size and there will be frequent page breaks. I don't mind them for free books or library books, but I won't buy a PDF.
I seem to remember a thread which had screen shots, but I can't find it. There is a discussion about the 505 and PDFs on this thread.
https://www.mobileread.com/forums/showthread.php?t=39568