After making a PDF publicly available from my Google Drive, I needed to make some updates, but uploading a new version would create a new document with a new ID. The existing link would still point to the old version, not the new. Fortunately Google has this one covered. Just right-click the PDF, select "Manage Versions" and upload the updated PDF. That will replace the old with the new using the same ID. Google will keep the previous version around for 30 days, just in case you need it again.