I would suggest using the pattern matching workflow activity to extract those numbers. Pattern matching uses regular expressions (regex).

A couple of assumptions:

1. I'm assuming that the name of the document (Entry Name) in your example is John Doe Smith 12345.pdf

2. The regex I used is .*\s+(\d+).pdf - highly depending on the actual naming of the files you may need to adjust the regex a bit. You can also string regex's together using a pipe |.

Example: .*\s+(\d+).pdf|.*\s+(\d+).tif

Here is an example of multiple regex's being evaluated in a single regex:

Then add the Assign field workflow activity and assign the pattern-matching token to your desired field.

Good luck!