Steps how to reproduce with PDF Viewer 2.2.255 and unzipped unofficial (by wg9s) De SeaMonkey 2.53.1 (NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build 20191207130010 (Default Classic Theme) on German WIN7 64bit:
Probably related to my User Profile, at least the problem is not reproducible with a newly created user profile and newly installed PDF Viewer 2.2.255.
Steps how to reproduce with PDF Viewer 2.2.255 and unzipped unofficial (by wg9s) De SeaMonkey 2.53.1 (NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0 Build 20191207130010 (Default Classic Theme) on German WIN7 64bit:
Open https://braunschweig.nahverkehrsdaten.com/download/neuesnetz/Fahrplan-2019_Grundlagen_Web.pdf
Expected: shown completely Actual: most contents missing