Is the PDF an "image" file in content?
If there is no text in the pdf - in effect the pdf file is a picture - there is nothing for Monarch to work with.
You can try converting it to text in Adobe Reader or one of the other tools that offers PDF text file interpretations of some sort to see what you get.
My guess is that it will be an empty document or something that is mostly empty but may have some seemingly random characters scattered around.
If that IS the case then you either need to go back to the source to discover whether the pdf file can be presented as a text based file rather than a graphic OR consider you options for first using an OCR tool to scan the pixels and work out what the text might be.
I assume you are not seeing an error message related to the file. Just no text when you expect to see something.