"Is there a way to include Alt-text without adding tags?"
Does anyone have an idea why a large part of the page disappears or what I can do to prevent it.
This is can be a problem particularly with complex graphics with lots of elements.
If you don't need to keep the document at high resolution (that is it's primarily for screen viewing), I would recommend copying the graphics using the graphic snapshot tool. Then Create a PDF from clipboard and save the resulting PDF as a PNG or JPG. Delete all the pieces of the complex graphic from your PDF using the Edit Text Tool to select them. Then place the PNG or JPG image in it's place (Edit Object tool -right click on the page - place image). Resize as necessary and manually tag that. It's a few steps but goes quickly once you learn how.
Or, you can simplify the graphics by exporting as PNG or JPG *before* you make the PDF. That's easy if you are using Illustrator to create them.
If you are making them in Word or Powerpoint, the above method for extracting images from the PDF may be your simplest solution.
This was suggested in another thread recently -- "when exporting the tagged PDF with 'Create Acrobat Layers' as an effective way to stop graphics dropping out. It was a different workflow (Indesign file with layers) but you could give it a try.
Thanks! Because of this advice I used Acrobat to delete every one of the drawing objets that Word had inserted, then tagged, and pasted back the drawings from another copy of the original file. Everything worked except two of the arrows are missing the triangles to indicate direction, and the image that I wanted to add Alt-text to is now tagged as Figure not ClipSpan. The options for the detailed description I wanted to add are not available for Figure. However, it is not worth the extra effort for me to try anything else now.
If anyone wants to dig into this deeper I'll tell you that: (1) it is not the original (multi-colour) image that seems to be causing the problem but rather the simple line drawings from Word; (2) When selecting objects in a region with Acrobat I saw a few very small rectangles that didn't seem to have any content. I guess that Word is somehow putting very large mostly empty graphics on a layer above the text and that Acrobat tagging is changing the background colour of those graphics to the background colour. But as I know almost nothing about Word and little about Acrobar it is only a guess.