Specific document protection

After scanning my machine, AVG gave a list of maybe 50 documents that it said were vulnerable. For example, one was a PDF of an invoice.

 then selected RESOLVE, and AVG said that it had fixed the issue.

What exactly did it do to protect this rather short list of documents on my computer?

Hello Marc.
Thank you for contacting AVG Community Support.
We understand your concern let us explain how the file will be protected.
Sensitive Data Shield is a feature in AVG Internet Security that scans and protects your personal sensitive documents against unauthorized access and malware. The files considered as sensitive contain your personal information which, if revealed, can compromise your privacy and identity. Sensitive Data Shield secures your private data by controlling which applications and users have access to your files. 
I hope this answers your question. Also you have posted a separate concern regarding buy button appearing on the program.
I request you try reparing AVG Antivirus (https://support.avg.com/SupportArticleView?l=en&urlName=repair-avg-antivirus&q=repair) and check on the issue.
Please feel free to reach us again if you need any other help with AVG.
We await your reply.

Hello Marc.
Thank you for contacting AVG Community Support.
We understand your concern let us explain how the file will be protected.
Sensitive Data Shield is a feature in AVG Internet Security that scans and protects your personal sensitive documents against unauthorized access and malware. The files considered as sensitive contain your personal information which, if revealed, can compromise your privacy and identity. Sensitive Data Shield secures your private data by controlling which applications and users have access to your files. 
I hope this answers your question. Also you have posted a separate concern regarding buy button appearing on the program.
I request you try reparing AVG Antivirus (https://support.avg.com/SupportArticleView?l=en&urlName=repair-avg-antivirus&q=repair) and check on the issue.
Please feel free to reach us again if you need any other help with AVG.
We await your reply.