You will need your data file back up and not your PDF backup if you are asking how to restore your client info from 2019. I'm not positive what your question is.

My laptop crashed; however, i had the 2019 PDF backed-up on a external drive. But, from the other responses, it's not possible to back ProSeries PDFs into the software. 

You already asked in another post.  You cant use a PDF to transfer into the ProSeries tax file.

You didnt back up the 19Data folder to the external harddrive as well?  Only the PDFs?

Correct, you can not restore your homebase or client files with a PDF. Set your external hard drive to back up your prowin XX or basewinXX files.