Hi Doeke,
Thanks for reaching out, I'd be happy to take care of this for you. This generally happens when the deafult app to open a PDF file is Adobe Illustrator. In order to change that, please change the deafult app to Adobe Reader DC or Acrobat Pro DC. You can make the changes from the same place shown in the screenshot.
If you have any further questions, please let us know. We'd be happy to assist. Thanks!
Regards
Rishabh