A group of Microsoft Products and technologies used for sharing and managing content, knowledge, and applications.
Dear @Maria Z,
Thank you for posting your question in the Microsoft Q&A forum.
Based on my understanding, you have a Document Library in SharePoint, which contains a list of PDF files. The ABC group is the group that has access to the site; you want to share an individual file with another user who is not part of ABC but is still within the tenant.
Have you tried re-adding them and re-sharing the link with them?
Also, let me ask, have you added them to the site yet? If you don't add them to the site, you cannot share files from the site.
Please help me go to Settings > Site Permissions > Advanced Permission Settings > click on a group > in the URL at the top, please help me change the end to =0 > remove the affected user > then try re-adding them.
Also, I want to ask, has that user ever been deleted and then restored?
Looking forward to your reply!