PDFs can be tricky to make accessible.  Regardless of whether you are using a Mac or PC, check out the instructions on the Adobe site for making a PDF accessible by running the action wizard and full check.  An all text Pdf should be relatively easy following the instructions in the first two sections at the Adobe site.  If after running the accessibility full check as outlined in the second section there are no issues identified, then the document is accessible.
(It is possible on an all-text document that Logical Reading Order and Color Contrast will have a white question mark in a blue circle.  It is ok to “pass” the Logical Reading Order.  And if it is a white document with black text it is ok to “pass” Color Contrast.)

If there are images, charts, tables, or maps these steps become more problematic, but the instructions in the remaining sections at the Adobe site should help with issues like alternative text and tagging issues.