One of the best Open Source PDF viewers on the Google Play Store has to be EBookDroid - PDF & DJVU Reader. It supports the formats PDF, DjVu, XPS (OpenXPS), Comic books (cbz,cbr), FictionBook (fb2, fb2.zip).
EBookDroid will open just about every book format, and displays them smoothly and quickly with no lag. It has a very easy interface and lets you access folders on your system without too many "import" features. Books are displayed perfectly and EBookDroid has lots of customization options both general and related to single books.
Using this app to read mostly PDFs and this app is the best so far, one of the best features would have to be how it remembers what page was last viewed across all the books that have been viewed.
Get EBookDroid from the Google Play Store