These examples use Adobe Acrobat Pro 8 for Mac. Other versions of Acrobat Pro may not function exactly the same, although it will be similar.
Begin by selecting the TouchUp Object Tool. You can select this tool using the menu Tools > Advanced Editing > TouchUp Object Tool.
Alternatively, there is an Advanced Editing Toolbar, which you see in the Advanced Editing menu options above. If you display the toolbar you will see all the Advanced Editing Tools.
In the Advanced Editing Toolbar, the TouchUp Object Tool is on the far right.
Now that you have the right tool active, select the image. Right-Click (or Control-Click) and select Properties.
In the Properties dialog box, you’ll find an alternate text field in the Tag Category.
Enter the alt text and finish up.
See also Tagging a PDF, which is several years old but still valid. A properly tagged PDF document will help screen reader users navigate more effectively.
For further information and for tips for other tools, see the W3C’s PDF Techniques.