I have the 7" Samsung Galaxy Tab (amongst other tablets). Usual reader apps that I use for that one are FBReader (for ePub documents), Kindle (for Amazon) books and ezPDF for PDF's.
PDF's are not brilliant on 7" tablets,
unless you are specifically getting PDF's formatted as novels. Textbooks and magazines normally have text that is too small - so you have to zoom the page and move around. I find that awkward (and use a 10" tablet for the purpose).
Aldiko (which I used to use a lot) has a fixed spot in the internal flash memory for books. (usually a folder called ebooks) as does the Kindle software (called, oddly enough - Kindle). FBReader can be directed to any folder you like, and I direct it to my MicroSD card - in a directory of my choosing. FBReader is also capable of looking inside ZIP files for EPubs - handy if you use Baen books a lot.
I use Kindle to buy books from Amazon - which I read across a range of Android, IOS and Windows devices. ePubs etc, I sideload from a PC connection.
The Galaxy Tab is a really neat reader - been using one for around 9 months now and like it immensely.
Ross