Text garbles after combining PDFs - Acrobat Pro DC

When I combine previously created PDFs, certain individual letters/numbers, on occasion, changes to boxes.  Sometimes its the whole word or number, other times its just certain letters, numbers, or punctuation.  I am generating PDFs in Autodesk's AutoCAD 2015.  The individual PDFs are fine, its just after combining them that they start to substitute boxes for the individual letters & numbers.  I have tried remaking the individual PDFs and recombining them, but it is the same problem.