get it in epub if you can. PDF is a crappy, frozen format unless you need the specific layout control it brings.
I have a program that converts PDFs to text, in case PDFs become unreadable in the future.
PDF is an open format with open-source software to read it (such as libpoppler). I don't see how they could become unreadable in the future, any more than JPEG could.
And for this same reason, PDF is quite ill suited for small screens like smartphones or tablets.
I prefer ePub for the reflow of text that adapts to the screen size and allows the user to select fonts and other reading settings (like spacing, justification, etc.).
Pdfs make maths work, epubs look absolutely horrible.