An image PDF is a snapshot or scanned image of your document. An image PDF is not accessible. Since it is merely a picture of the document, the text and content of an image PDF will not be translated to assistive technologies like screen readers and text-to-speech software.
To determine if the PDF is an image, try to select individual words from the document. If you cannot select any individual words or letters and the cursor draws a blue box on the page, the PDF is probably an image PDF. All image-only and scanned image PDFs need to be converted to a readable PDF by performing text recognition.
Readable or searchable PDFs are PDFs which have been optically scanned to recognize individual characters of text. You can use Adobe Acrobat Pro to run an optical character recognition (OCR) to perform text recognition. Readable documents are able to be edited, searched, and assistive technology will "read" the recognized words in the document.
A tagged PDF is formatted with document tags designating specific types of content (i.e. headings, paragraph text, images, tables) allowing individuals who use assistive technology the ability to scan and navigate a document's content. Document tagging will not change the visual design of your PDF. Instead, it will provide assistive technology users the needed information about the document's structure based on the tags.
To determine if your PDF has document tags, open the Tags Pane on the left side of the screen.