Cannot open "downloaded" pdf on Android since another app named AIRNAV PRO present on my mobile supersedes and takes on the opening and fails opening. Other pdf present on the device however don't have any opening issue.
Looking at the description given above, it seems that an another PDF viewer named AIRNAV have taken over command and set as default PDF viewer to open .pdf files.
If yes, then the easy and best possible solution is to uninstall this app. If you are not familiar or you didn't install it intentionally then better to take it off.
Once done, try opening a .pdf file, you will be asked to select the available pdf viewer on your phone. Allow once or use this app. There you can set Acrobat Reader mobile to your primary PDF viewer app.
Similarly like explained in this help article with detailed steps: https://www.adobe.com/devnet-docs/acrobat/android/en/openfiles.html#open-email-attachments
Hope this helps!