KOReader is a document viewer for eink devices. The formats it supports include PDF, DjVu, XPS, CBZ, FB2, PDB, TXT, HTML, RTF, CHM, EPUB, DOC, MOBI, and ZIP files.
To me, it's THE best PDF reader on NGP (because it reflows scanned PDF files).
Here the method to install it on NGP which I have personally tested:
1. First of all, you need to root NGP and put a folder called "koreader" under root.
2. Extract attached "patch.lua" file under "koreader" folder.
3. Install this stable version of Koreader.
https://github.com/koreader/koreader....11-stable.apk
4. Restart NGP (you may need to restart more than once if it shows black screen).
You can also change fonts of Koreader here:
data/data/org.koreader.launcher/files/fonts/