If a PDF copy of an invoice is not able to be viewed on a PL transaction it is possible to add it in PL Miscellaneous Options and Edit Transactions Details
Enter the account number and find the transaction, click on the ellipsis at the right of the Document archive ID and there will be an attach option to allow you to select a PDF. (If the attach option is not shown please contact Systime Support to have it set up)
Clicking the attach option will allow you to search for the PDF on your PC
Select the document and save the record and you will be able to view or print it from the PL