Add an option to the right-click context menu on PDF tabs to 'Pin to File Access'.*
Currently, you have to find the desired PDF in the Recents list. The desired PDF may be buried down the list, depending on how many files were opened after it was. It may be difficult to be certain of the correct file, if filenames are similar; While open PDFs are bolded, there doesn't appear to be any indication in the Recents list of which open file has the current focus.
Adding the Pin option to the tab context menu would be a vast improvement in the ease of adding PDFs to the pinned files.
*Also consider adding 'Email' as well…
