The basic requirement for a pdf file which you know contains text information (not just graphics data) but cannot be read is that you try a re-write of the file to see if the new version is more readable. The problem is that there are now many pdf file writing programs but not all are consistent in the rules they apply and indeed not all documents have been been well structured to start with.
Using Acrobat to open and then save the file can be expected to create a new file which does obey the Acrobat standards - or at least a file which obey standards that Monarch will recognise! In effect it takes the components back out of the original file and starts again.
Any other program that can deconstruct a pdf in its entirety (not just seek to extract the text it contains) and then creeate a new file form the result should be able to do the same thing.
Provided it was not that program which created the uninterpretable file in the first place, you would likely get a good result.
One alternative would be to recreate the pdf file using a different pdf writer program.