Date created incorrect in PDF's

I create PDF’s in PDF Pen Pro by combining several other documents into one and save into DTPO. Sometimes, however, these arrive into DT with the created date as 01/01/1970. I presume this means it has no “created date”? Any way of stopping this behaviour so I don’t have to namely go in and Get Info and amend the Created Date?

Thanks.

What’s the creation date of these files in the Finder?

Also show as created on 01/01/1970! Presume this is a default date when there is no metadata for this?

This behavior is sometimes seen when there are problems/corruption with the hard drive. As a precaution, it might be a good thing to verify/repair the drive.