MuPDF is outdated
F-Droid Version 1.7
https://f-droid.org/repository/browse/?fdfilter=mupdf&fdid=com.artifex.mupdfdemo
New Version 1.7a
http://git.ghostscript.com/?p=mupdf.git;a=log;h=refs/tags/1.7a
New MuPDF 1.7a release.
Uses less memory, runs even faster, and now handles epub (v2, no-DRM) files too!
( https://play.google.com/store/apps/details?id=com.artifex.mupdfdemo )