I am facing Duplicate without user-selected canonical issue in pdf file

Hello,

Thanks for contacting us, and sorry for any inconvenience that might have been caused due to that.

When you upload media like images or pdf to a post, an attachment page is automatically created. The attachment page for them is also the origin URL you want to redirect from.

Please note that the attachment pages are no longer recommended to be indexed and as such no SEO has to be done directly on them. You can check this article for further information: https://www.seoblog.com/wordpress-attachment-hurt-seo/

In this case, you can redirect those attachments by following the steps in this link:
https://rankmath.com/kb/should-you-redirect-attachments/

Once done, clear your website’s cache, revalidate the issue, and give Google some time to recrawl your site.

Hope that helps, and please do not hesitate to let us know if you need our assistance with anything else.

Thank you.