Multithreaded so the UI is always quite smooth and responsive.
Ability to print the PDFs.
Ability to email the PDFs.
Add bookmarks to your PDFs to save your spot.
iBooks-like document navigation.
Device rotation and all orientations.
Encrypted (password protected) PDFs.
PDF links (URI and go to page).
PDFs with rotated pages.
PDF left and right page navigation.
PDF multipage overview.