Error Message when Combining Files to PDF in Acrobat XI-Pro

Hi!

I am trying to combine multiple CAD files to PDF and keep encountering an error message.

I'm using the Combine Files to PDF, dragging in the corresponding files (.DWG files) into Acrobat and then clicking the Combine button. They convert to PDF but a dialog box immediately pops up:

As I'm the person creating the .pdf document and am using Acrobat XI-Pro to do so, what am I doing wrong to continue to receive this message?

Thanks for the help!