If a PDF document contains untagged or unmarked elements, you can locate them in the logical structure tree and tag them by using the Find command. Steps are as follow:
- Click the icon
or 
on the navigation pane to open the Content panel or Tags panel. - Click the Options menu at the top of the panel and choose Find, (or right-click on any element in the panel, and choose Find from the context menu).
- In the Find Element dialog box, do the following:
- Choose the element you want to find in the Find drop-down list.
- Select where you want to search the element: page or document.
- Click Find Next to search for the unmarked elements.
- Click Tag Element to tag the unmarked element. And then click Close upon completion.