I am facing this issue where a secured file can be annotated on Acrobat Reader on Windows but not on android version of Acrobat Reader.
I am running the following
|OS||Adobe Reader version|
|Windows 10 (1607)||2017 Release | Version 2017.009.20044|
|Android 7 on Tab S3||17.1.1|
The file permission seen on Acrobat Reader on Windows.
The error message displayed on Acrobat Reader for Android when I try to annotate the same file.
In this case, I can clearly annotate the same file on Acrobat Reader on Windows so permission issue can be ruled out.
I suspect this might be a bug on Acrobat Reader on Android and hope fix will be released soon. I read pdf files on my tablet and the ability to annotate is important for me. Thanks.
It seems that the document is encrypted because of the document security and restrictions settings.
Please refer this thread with similar issue: Restricted Document
I hope this will answer your query.
Let us know if you need any help.