pctechtv: some suggestions, depending on what exactly you want to do with the PDFs.
The best way, of course, is to place the metadata inside the PDF, and the official way to do that is using Acrobat Pro DC;
Get the complete PDF solution US$14.99/month
-this gives you complete control (inserting, editing, saving PDF/XMP metadata), but at an ongoing cost.
Less expensive is PDF Architect at:
http://www.pdfforge.org/pdfarchitect/modules
Modules included in this package: Convert, Edit, Insert US$59.00 one-time cost.
Note: you can use the PdfCreator for free, and it will allow you to set some metadata as you print-to/create the PDF.
More technical, but free, open-source is: ExifTool by Phil Harvey
http://www.sno.phy.queensu.ca/~phil/exiftool/
Read, Write and Edit all Meta Information. Windows, Mac OSX executables available.
ExifTool is a "command-line" utility. That means, there's no Graphic User Interface (GUI), so all work must be done by typing commands inside "Command Prompt"
You could make DOpus buttons to do things, but I found this easier and more flexible: ExifToolGUI
http://u88.n24.queensu.ca/~bogdan/
a very useful GUI graphical front-end to ExifTool
with these you can add/edit the important metadata in PDFs, like (these are ExifTool commands):
Set PDF metadata=-PDF:Title="my Title" -PDF:Subject="my Subject" -PDF:Author="myAuthor"
Set PDF mod.date to value=-ModifyDate="2015:07:01T12:00:00-07:00"
update PDF file dates using XMP dates=-FileCreateDate<XMP:CreateDate -FileModifyDate<XMP:ModifyDate
Since I don't know of any way in DOpus to view/edit the internal XMP in PDFs, I have created a button to call ExifToolGUI for the current folder, and operate on the PDFs that way.
Maybe Leo knows a way to use the powerful scripting in DOpus to edit the internal XMP in PDFs. Or, maybe PDF metadata view/edit is a feature coming in DOpus12?