Error: "Cannot open files due to an Unknown Error" in Illustrator and Error 89 in Acrobat

Hi,

I would like to know if the steps suggested above worked for you? If not, I would request you to share a few more details along with the ones asked by Monka: 

• Exact OS & version of Illustrator
• Is the issue related to a specific file. If yes, could you please share the document with us? You can upload it to the Creative Cloud (https://helpx.adobe.com/in/creative-cloud/help/share.html) and share the download link.

You may also try the suggestions shared on this similar discussion (https://community.adobe.com/t5/illustrator/error-acrobat-pdf-files-is-having-difficulties-file-does-not-begin-with-pdf/m-p/9840087).

Kindly update the discussion if you still need assistance with it. We'd be happy to help.

Thanks,
Anshul Saini

Which version, which system?

Where is the file located?

What is the file's history? Was there a crash the last time you saved it? Did you get it from someone else?

How large is the filesize?

Are you running off a network?

If you'd typed in "acrobat pdf file format is having difficulties" as your subject you would have been pointed here:

https://community.adobe.com/t5/illustrator/quot-acrobat-pdf-file-format-is-having-difficulties-quot-error/m-p/10215458