As with other online resources, its our responsibility to test for accessibility, and you should never assume a PDF, even one that looks very professional, is accessible.

There are different ways to test the accessibility of your PDF, you could use:

Adobe Acrobat's Accessibility Checker (or a free tool like PAC 2.0), or
a screen reading software like Jaws and NVDA.

If you do not have access to screen reading software, or don't know how to use it, you can use the Accessibility tool in Adobe Acrobat Pro. This tool will also allow you to modify the PDF and correct any accessibility errors, if the document is not "locked." To address the accessibility issues:

Open the file in Adobe Acrobat Pro.
Run the Accessibility Check.
Follow the tips generated by Adobe to fix any issues.

There is a caveat. Since source material curated from others is protected under copyright, you need to ensure you aren't changing any content or formatting that is part of the copyright. This means you shouldn't remove any logos or images, even if they are not accessible, nor should you remove or change any text. Retrofitting someone else's document could be a can of worms...

If the document is not accessible, unless the resource is amazing, fantastic, and irreplaceable, your best bet may be to create the resource on your own, or look for a better resource. And, finally, at the risk of sounding like a broken record, it's a good time to remember that when we use documents from other sources, we are not only responsible for ensuring the document is accessible, but also for ensuring we are complying with copyright and fair use. If you pulled the document from an internet source, check to see if it has a Creative Commons license. In all cases, model the ethical use of materials for your students by including a citation and a link to the original document, when appropriate.