Hope you are doing well. Sorry for your experience with Acrobat Reader Mobile.
For the first part, those are the boundaries that the application should have. This is not limited to Acrobat, but to all the applications you might have observed. This is to keep the aspect ratio of the applications intact.
For the second part, is it related to a particular file? I tried reproducing this on my end, and everything worked fine. Could it be a case where the page numbers are put in manually? I tried with files saved on my local drive and shared files, but everything was fine.
If you experience the issue with all the files, please share the below pieces of information so we can investigate further:
1. The application version of Acrobat Reader Mobile you are using;
2. The OS [Android/iOS] and its version;
3. A screen recording of the issue.
Also, you might want to check if the application is updated.
Look forward to hearing from you.