1.3.2 Meaningful sequence
This guideline says:
When the sequence in which content is presented affects its meaning, a correct reading sequence can be programmatically determined.
What does it mean?
Most things are read in the order that we see them. If you create a Word document, a screen reader will generally start at the top of the page and read it all in order to the bottom of the page. However, think about a PowerPoint presentation. This is not just a top to bottom reading of text and images. Things could be anywhere on the page and the reader generally uses visual clues to help know what order to read it in.
I don't know about you, but when I create a PowerPoint presentation (and this equally applies to other slide-based apps, like Storyline), I create text, then copy and paste bits to amend and move around the slide. Then I might add pictures and move them around. If I do nothing else, a screen reader will read it in the order I created it... which is rarely the correct order!
Another issue that affects reading order is how you create. I often get presentations sent to me to be checked for WCAG compliance before sending out or converting to PDF. I find many people don't realise you can type into a shape, so they create a shape and then create a text box and put it over the top. Now both of these will be read by the screen reader.
How do I set the reading order?
In theory, you should be able to get all the accessibility stuff right in PowerPoint and then convert it to PDF and the PDF should read okay. However, I would always suggest checking it with JAWS (you can get a trial version) or with the built in Windows 10 narrator. Never assume that your PDF has kept everything correct.
If you need to correct a PDF, you will need a professional version of your PDF software. I use Adobe Acrobat Pro. This has a tool for checking and enables you to do some fixes.
Where possible though, I would usually recommend making the original PowerPoint available instead of or as well as a PDF.
Working with PDFs to improve their accessibility is one of my next professional development activities. I can do basic stuff but I need to be better at it and want to lose the feeling of panic I get when I see a PDF coming my way.