I have almost 4,000 PowerPoint slides from which I am asked to make accessible PDFs. I'm working with PowerPoint 2010, and creating the PDFs using the Acrobat X plug-in as I am told that's the "best" way to make accessible PDFs.
Many of the slides have "PowerPoint art," sketches made by putting shapes, arrows, etc. on the slide. I've found that I can use "Arrange -> Group" to make an object of the collection of shapes, and I can assign alternate text to the object with [right click] Format Picture -> Alt text. However, when I then choose Acrobat -> Create PDF the alternate text does not appear as a tool-tip, as it does on images. (I know of no way to test whether a screen-reader can see it.) Edit: OK... I've figured out "Read Out Loud," so I can have Acrobat read to me. It's slow, but it will let me check. To my dismay, I've found that Acrobat X and Adobe Reader X read differently, with Reader X reading some things that are tagged as background.
Further experimentation shows that I can use PowerPoint's "Save as Picture" to make a PNG of the sketch, paste it into the slide, and apply alternate text to the image. This is hideously labor-intensive and leaves something that cannot be edited with PPT. If I paste the image over the PPT art, any alternate text originally assigned to the underlying shapes appears still to be there, which is likely to be confusing to the blind user.
Surely there is an easier way to do this, but I haven't been able to find it.