Are the PDF's fonts embedded? If not re-create the PDF using a Distiller job option that embeds fonts (e.g., High Quality Print).
When changing font the desired font must actually be installed on the computer and available for use.
Acrobat relies on the PDF creation process having added correct font encoding tables that map properly to UNICODE.
This may not be the case with your PDF.
If you place the PDF in a file share online (files.acrobat.com / dropbox / onedrive / google drive / etc) and then post the "share" link here someone may have the time to poke about within the PDF and provide some suggestions.
Otherwise it's just an easter egg hunt at this stage.
Be well...