Yes, trying to open in Acrobat Reader, or even trying to rename the file is not allowed. The message from Acrobat is "There was an error opening this document. This file is already open or in use by another application." If I try to rename the PDF the resultant error message names the Xojo app as the one that has the file open. All of this goes away when I exit the Xojo app.
Saving the file on a shared volume, I can then see the file locked status which says it's locked for Write.
Doing some more testing I found that this only occurs in 64-bit Windows apps. 32-bit works fine.
The problem does not appear to be Xojo itself, as I can create a file and write to it with BinaryStream, but still access/rename/delete that file without exiting the Xojo app.