No Administrator ID

My friend Frank recently migrated from Windows 8 to Windows 10. In the process he ended up having to login via a Microsoft Account.
But he was having weird problems, so as an experiment I converted him to a local account, cunningly named "Frank".

ID Frank has Administrator access, according to the User Accounts. But there are some files on his system that he cannot access. If he goes into the Security tab of one of these files, instead of getting a list of accessors, he gets a popup message "You must have Read permissions to view the properties of this object..."

It seems that he doesn't have read permissions...

So, since he has Administrator privilege, I tried to take ownership of one of these files. But there is no "Take Ownership" action that I can find.

How can I help Frank get access to these files?

The plot thickens after this point, but let's see how we get on so far.

Any files owned by the MS account he won't have rights to

Well, I said the plot thickens... Whilst he cannot see the security tab (apparently he lacks read permission), he can open the word documents in Microsoft Word. Odd, eh?