Unfortunately no, an associated PDF can't be attached to a Paperless Office Journal and Register log record after the fact.

I will take this idea and see if a future enhancement to allow doing that can be implemented. 

There is a workaround that could be done.  Using DFDM the associated PDF information could be added.  Using DFDM should be done with great care as using the utility may damage data files.  What version of Sage 100 do you have installed?