Would addingAdding a time stamptimestamp to the file name of the generated PDF would help? That way if the user makes a change and then goes to copy or send the file right after, they might notice that the time stamptimestamp is older than the last change they know they made.
Example: instead of:
invoice-10043-acme_corp.pdf
Consider something like:
invoice-10043-acme_corp-exported-20190706T0944.pdf
There are different ways of formatting the timestamp, but I would encourage using one (like this) where if the user sorts the list of files by name, they are sorted properly by time stamp as well - that would help them build the habit of always looking for the most up-to-date one on the server.