Google will roll out an update beginning September 13, 2021 to make sharing links more secure, by adding a resource key. Affected files include non-Google files (PDF, Word docs, etc) stored in My Drive or Shared Drives prior to November 2017.
If you own or manage any files affected by this change, Google will notify you by email beginning July 26, 2021. The email will provide instructions on steps to take and will include a link to view a list of impacted files you own or manage in Google Drive.
On September 13, the change to the links may result in access requests from individuals who have not viewed the files previously but are trying to access them using a link. If you are able to replace old share links with the new links after you are notified, you can reduce the number of individuals who will need to request access.