The first step in creating an accessible PDF from PowerPoint is to ensure that the original slide deck is accessible. Make sure to follow the core steps for accessibility as outlined on the PowerPoint presentations page.
Starting with an accessible slide deck, a goal when exporting to PDF is to do so in a way that preserves the structure of the PowerPoint, including slide titles, reading order, alternate text for images, and markup that explicitly identifies lists, tables, document language, and other content that is important for accessibility.
Techniques
Do not print to PDF. This method will not preserve the document structure and will result in a PDF document with no tag structure.
PowerPoint for Windows
From the File menu, select “Save As…” and choose where you want the file to be saved. In the Save As dialog box, select PDF from the Save as type list. By default, this produces a PDF that preserves the document structure and assures accessibility.
PowerPoint for Mac
Users of PowerPoint for Mac should be cautioned that exporting to PDF will not yield an accessible, tagged PDF document, it must be remediated using Acrobat Pro.