SECRET AGENTS wouldn't load or export to musescore, it is only 16 pages

As it says "Please don’t try to convert the same PDF file twice. If it doesn’t work, your PDF file isn’t compatible to the converter".

Anyway, he website gives tips and I tried, it failed. There is no way going forward with the online PDF interpreter. You could try downloading the Audiveris or other PDF " conversion" software to your PC (or Mac or whatever system you run) but I still would doubt that you would get anything useful.

Import, not Export...

"... an error occurred, how do I fix it?"

1. Accept that the optical music recognition (OMR) offered by MuseScore with File > Import PDF is not optimised and has not been updated or improved in a long time.
2. Either you can download Audiveris for free and install it on your own computer (Windows or Linux):
   https://audiveris.github.io/audiveris/_pages/install/binaries/
   Then you can experiment with the Audiveris settings to get better OMR results.
3. Or you can investigate alternative free OMR systems to install on your own computer. See Wikipedia:
   https://en.wikipedia.org/w/index.php?title=Optical_music_recognition#Ac…
4. Or you can look into the options for running a commercial OMR app on your mobile phone. See Wikipedia:
   https://en.wikipedia.org/w/index.php?title=Optical_music_recognition#Mo…
5. Or you can buy a commercial OMR application to install and run on your own computer. See Wikipedia:
   https://en.wikipedia.org/w/index.php?title=Optical_music_recognition#Co…

The attached MusicXML file is the result of capturing your PDF with a commercial OMR app for PC (Photoscore). The file is exactly "as generated", without any post-capture corrections. Pages 12-16 are blank, as they are on the original PDF.