Before you can create a tagged PDF, you must first ensure that the scanned PDF contains real text and is not simply an image. Many scanned documents are just snapshots of the document that has been scanned, and the document does not contain real text. If the document does not have real text, consider either reconverting using Adobe Acrobat Pro (if you have the original file) or convert it to text via OCR software such as OmniPage.
If you do not have OmniPage, the following information might be helpful:
To convert the document to text via a scanner, choose Document > OCR Text Recognition > Recognize Text Using OCR. The quality of the text depends on the quality of the source document. In other words, if the scanned document includes writing in the margins, it will be challenging to scan to OCR because the handwritten text will be recognized.
You can attempt to clean up the document by selecting Document > OCR Text Recognition > Find All OCR Suspects.