The current PDF viewer integrated into the Android version of Proton Drive is very insufficient for viewing PDF files.

I would like to have an option to open PDF files in another app when accessing PDF files in my Proton Drive.

See: "Open in another app" feature in the Android version of OneDrive.

Currently a way around this is to download the PDF file locally, then navigating to the Adnroid File Manager, finding the downloaded file and opening it in a PDF viewer of your choosing.