When people talk about "accessible" PDF files, they usually are referring to "tagged" PDF files, even though there is more to an accessible PDF than tags. PDF tags provide a hidden structured, textual representation of the PDF content that is presented to screen readers. They exist for accessibility purposes only and have no visible effect on the PDF file.
To check whether a PDF is accessible, try selecting text within it. If you can select and highlight text, than the PDF is discoverable by screen readers. If not, then the PDF should be converted to an accessible format.
Adobe Acrobat is the most common software for creating accessible PDF’s. You can also do this in Word. The procedure is well described in Web AIM’s PDF accessibility page.