I tried replicating the issue at our end with security added to the pdf (document open password - 1234 and change permission password - 12345) with 256-bit AES encryption level in Acrobat, tried to open the same file in Acrobat Reader mobile app using both the passwords, and it opened without any issue. You may need to contact third-party vendor and see what they have to suggest.
Like the prior respondent above, I have been unable to view my digitally signed final transcript in the Adobe Reader Android app as of late, although I definitely could do so in the 6+ months immediately after my graduation in May 2015.
I must say I am disappointed that Adobe apparently never bothered to read my low app rating on the Google Play store. When I graduated my Master's program in May 2015, your then-current Android app at the time allowed me to view my digitally signed final transcript just fine. Within the year, I was no longer able to do so and that matter has continued unresolved since.
I am including a screenshot from this morning to confirm that the issue remains unresolved as of Android 8.1 (February 2018 security patch) on my Nexus 6p primary device. I am also including the file itself for your reference, since you apparently never bother to test such files yourselves without a specific directive to do so from a dissatisfied client. I downloaded your latest update from this morning beforehand, so do not bother to ask me to do so in any follow up correspondence.
If you intend to maintain PDF as a dominant file storage medium, the least you can do is to ensure is that your encryption and security integrated options within the file format are supported!