If a PDF has no tags, there are several methods you can use to add tags with Adobe Acrobat. We will cover two of those methods here.

The Undo function does not apply to tags, so we recommend that you save a backup copy of a document prior to editing its tags.

1. Open the untagged PDF in Acrobat.
2. Open the Tags panel.
   
   If the document is untagged, you will see a message that reads "No tags available."
   
3. Click the options menu (or right-click the "No Tags Available" text) and select "Add Tags to Document." Acrobat will automatically generate tags for the document.
   
4. Please review your tags in the Tags panel and make corrections if necessary. The Add Tags to Document function often misinterperets the PDF's contents and applies tags incorrectly.
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1. Open the untagged PDF in Acrobat
   
2. Open the Order panel. Click the Options menu and select "Show reading order panel."
   
3. In the preview pane, click and drag to draw a marquee around the first piece of content (This could be an image, headline, text, etc.) in the document. Your marquee must completely enclose a piece of content in order to select it. If you accidentally select more content than you meant to, hold the Alt key (Windows) or the Option key (Mac) and click and drag to draw a marquee around the content you wish to deselect.
   
4. Choose the appropriate button for that piece of content from the Touch Up Reading Order panel. Although the list of options isn't identical, these buttons mostly correspond to the list of standard PDF tags.
   
5. Repeat steps two and three for the rest of the document. As you work through the process of touching up the reading order, tags are automatically added in the Tags panel.
6. Please review your tags in the Tags panel and make corrections, if necessary.
   

Method one may seem faster than method two, but the automatic Add Tags to Document function often makes mistakes that take a lot of time to correct. Results will vary from PDF to PDF. Method two requires more work up front, but it generally creates a more reliable tag structure that requires less revision afterward.

Regardless of the method you choose, you need to review your tags in the Tags panel and make corrections as necessary. Check to verify that the use of tags is logical and consistent, and that tagged elements are properly sequenced. Refer to the Standard PDF Tags guide to determine if the correct tags have been applied. 

In the Tags panel, you can click and drag tags to reorder them or change the nesting order.

Right-click a tag and select Properties to change it to a different tag type or give it a title.

You can create a new empty tag by clicking the options menu (or right-clicking a tag) and selecting New Tag.

Don't forget to add alt text to images, if necessary.