At the most basic level, a PDF must contain text rather than being an image of text. A PDF that is considered to be "Image Only" does not contain underlying text - it is simply a picture of text.
However, this does not insure reading order, logical flow of information, or access to PDF form fields.
Tagged PDF documents provide best option for maintaining reading order, flow of information content, and access to PDF form fields. Tagged PDF documents result in accessible and usable PDF files.