@RafaelNadal Hi, since you mentioned the app is available on the Play Store, could you provide the link since I didn't find it? OTOH, I found app made by Sigrid that's related to education. If that's the same app, then I think the PDF file is DRM'ed to prevent redistribution, including opening with other PDF readers.

@RafaelNadal You can double-check whether the .pdf is DRM'ed or not by either checking the file properties on a PC (security tab, if it states 'no security' then no DRM), or by opening the file through a text-editor and search for hits for "encrypt" (if you get any hits, likely DRM'ed).