Accessible PDF documents
There are multiple ways of creating an accessible PDF document. You will likely need to use a combination of the following tools / software / resources to create an accessible PDF document.
Microsoft Word
PDF Preparation
It's always best to start with the source file if it's available. If you have the original Word document, this allows you to leverage Microsoft Word's Accessibility Checker, where the interface is much more user-friendly and will minimize the amount of remediations needed in the final PDF document.
Below are AskUs articles on how to use Microsoft Word's Accessibility Checker and how to fix the accessibility errors commonly found in a Word document.

Checking and Addressing Accessibility Errors
Converting Your Accessible Word Document to PDF
Adobe Acrobat Pro DC
PDF Remediation
Even if your PDF was generated from the original source file with accessibility in mind (e.g. with Microsoft Word), your file may still contain PDF accessibility errors. You'll want to run Adobe Acrobat Pro DC's accessibility checker to review for any remaining errors.
Below are AskUs articles on how to run your PDF through Adobe Acrobat Pro DC's accessibility checker and how to address some of the common errors.

Checking and Addressing Accessibility Errors
TaggedPDF.com
TaggedPDF.com contains a large collection of articles that documents how to fix most PDF accessibility errors. Most articles also have a video screencast of the steps as well.
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