When opening PDFs from the Google Drive app in Adobe Acrobat, it uploads a new version of the file to Google Drive. It doesn't upload another file, but updates the existing file with a new version and therefore also taking up just as much space as there are versions of the file in a more hidden way, as it isn't immediately clear how many redundant versions there are of each file in Google Drive.
You also don't need to edit the PDF, which would have made sense to update the existing with a new version, but just opening and closing the file is enough, which quickly results in some files becoming quite large with many redundant versions each time you open it.

A scenario for me: Opened a PDF on my tablet, WiFi went off before it had time to upload a new version, updated the file on Google Drive from my computer with an actual new version, WiFi turns back on on my tablet, and it uploads the redundant file of the previous version as if it was an even newer version.
Also just the fact that you can't get a reliable timestamp for when the file was last modified to see if another person has actually updated it or just opened it in Adobe Acrobat.

I'm on a Pixel 3XL always running the newest version of stock Android as well as Adobe Acrobat, and this bug has persisted for several (at the very least 3) years as of now.