That's not possible unfortunately,
Google Classroom is related to Google and they use Google Drive to store files, so the arrow you click is actually a drive.google.com link and not a direct android media file share to select your Fav pdf viewer from.
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The close way would be to uninstall Google pdf viewer and disable Google Drive(it's system app most times, and has internal G-pdf viewer)
then from G-classroom open a pdf it will go to browser and hit download from drive page, and open it from notification after downloading, it's a bit faster method than download from drive and select manually from inside Acrobat of Download Dir.
That's not possible unfortunately,
Google Classroom is related to Google and they use Google Drive to store files, so the arrow you click is actually a drive.google.com link and not a direct android media file share to select your Fav pdf viewer from.
---
The close way would be to uninstall Google pdf viewer and disable Google Drive(it's system app most times, and has internal G-pdf viewer)
then from G-classroom open a pdf it will go to browser and hit download from drive page, and open it from notification after downloading, it's a bit faster method than download from drive and select manually from inside Acrobat of Download Dir.