Scanned images enclosed into PDF file, result image-based PDF file that has no text layer.

When converting such document to MS Office document the resulted file will consist of images only.

OCR (optical character recognition) function helps on scanned files (any image files) to have the text extracted.
For small amount of scanned document you may use free online OCR service, such as

If you need OCR function on daily bases, best is to own it. The latest Expert PDF provides OCR function for dozen of languages in the Ultimate edition. With Expert PDF Ultimate you can create editable text. If you own other edition of Expert PDF, order licence for the OCR module only or the entire Ultimate package within the software through Modules and Packages tab.