It may be that the simple act of making a change, any change, and recreating the the PDF will be enough - so long as the PDF compatibility allows older version of Adobe readers to understand the file - i.e. it's not restricted to be only compatible with the latest greatest version of Acrobat or the Reader.
Worth trying first. Also check if Adobe Reader can make a successful text conversion file from it.
If that doesn't work can you share the file so someone can have a look at it from a technical perspective? (I appreciatate that this is not always possible for security an privacy reasons.)