That is a more complicated situation - it would require that the vendor have macros enabled when they open the workbook, and keep them enabled the whole time. At some point, it just becomes impossible to ensure that they have done everything correctly, but one way would be to produce the sheet that they must print to the PDF by using a macro. And then you need to ensure that the sheet was actually produced by the macro and not just overwritten and then re-printed. I think that one way would be to create a new sheet by copying the output form, then inserting a new row into the print area and - using code - insert a date time stamp and a hashed value that is generated from the date using the macro. Lock the project so that they cannot view it - not great, but better than nothing - and then check that the hash value matches what you would expect given the date time stamp.

Or change how you work - don't accept a PDF, but require the originating workbook, and then send the vendor a PDF that they confirm as correct before proceeding.