Without having a copy of the PDF to do a look-see three activities come to mind as possible solutions.
Locate the item in the Content panel. with the content's container icon selected use the panel's Options menu to create an artifact.
Identify where the PDF page content fits in the content flow. Create symantically correct tag(s) (elements) in the appropriate location within the structure tree. With the tag selected use the Select tool to select PDF page content. Use the Tags panel's Options menu to Create Tag from Selection. The container icon for the selected content will appear as a child of the tag/element you had selected.
Locate the tag/element in the structure tree. Use the Tags panel's Options menu to Change Tag to Artifact.