If you open your PDF in Acrobat Reader, and look at File > Properties (I think), it should tell you the details of the version of the PDF standard that has been used to create the PDF, like PDF 1.0 or PDFa 1.1.... It should also tell you the tool used to create the PDF - e.g. Acrobat Distiller 1.4, or AmyuniPDFPrinter 2.3.
Check that the version of the PDF and the PDF creator are both as old as version 9 of Monarch - which was released in 2006, I think. If you are using PDFs that are created to a more recent standard, it's unlikely that v9 is going to cope.
It's worth downloading the free trial of Datawatch Modeller (it's part of the Datawatch Desktop package on their website) to test whether v12.4 can cope with your PDF. Or you can email the PDF to us at Monarch Experts and we can test it for you.
The only other thing to check is that you can open the PDF in Acrobat Reader and select all the text and copy and paste it into Notepad.