PAC 2 Warning
[X] structure element used as root element
(Note: [X] could be any structure element other than Document)
PDF/UA > Logical Structure > Structure Tree > [X] structure element
Matterhorn Protocol Checkpoint 01-006: The structure type and attributes of a structure element are not semantically appropriate for the structure element.
In the Acrobat Pro Tags pane, create a new Document tag and position it at the root (the top) of the tag tree. Select the rest of the tag structure and drag it inside the root Document tag.
Video version of this suggestion: