You can pre-render all pages in advance - and put these pre-rendered pages into cache so Adobe Reader will display them immediately ... regardless of page/file size ... if page is found in cache (/media/internal/appdata/com.quickoffice.ar/.cache/<hash>
It is possible to run script that will search your device for all PDF files and then render all pages...next time you open file in Adobe Reader - there will be no slow rendering, just immediate output (from cache).
There are few people that can easily write such script...
That can be added as an option but will take a long time to perform and take up a good amount of space too. I will look into it.