PDF documents contain more than just text or media content. This becomes apparent when checking the metadata of a PDF. One of the pieces of information you can find there is called Tagged PDF. It gives you information about the structure of a PDF, using the values “yes” or “no” to state whether the document is, in fact, a Tagged PDF or not.
Tagged PDF files can contain information that defines the structural logic of the document. This structure is used by PDF viewers and other programs that can open these documents in order to display the PDF correctly. It contains information about the importance and presence tables and other media, alternative text for images, the reading order, the presence of form fields, and the like.
For information about tags in image files, check out the Tagged metadata.