Validity of PDF for image description

I am talking about a site in which certain images contain mission-critical information. I was wondering: would a solution in which a PDF file is replacing an image description (and ALT) ever be considered accessible according to the WCAG 2.1 standard?

Would the acceptability depend on the accessibility of the PDF itself? If the PDF contains a screen-readable version, it will functionally be possible for the user to get the information, if not convenient. However, if the PDF fails the accessibility check, then it would naturally disqualify.

What do you think?