What does Text layer mean for your PDF?
A PDF made from Word, a browser or a design program stores real characters with coordinates: you can select a sentence, copy it, and a converter can rebuild paragraphs from it.
A scan holds a photograph of a page and nothing else. It looks the same on screen, but there is no text to select — which is why converting a scan to Word or Excel produces nothing until OCR adds a text layer.
The quick test: try to select a word. If the cursor sweeps a blue box over the whole page instead of the word, it is a scan.
How do you check whether a PDF has one?
Two seconds, no software. Pick a word you can see on the page and press Ctrl+F (Cmd+F), then type it. A match means there is a text layer. No match means you are looking at a picture of the word.
The second test is the drag: try to select a single word. Real text highlights word by word. A scan gives you a rectangle over the whole image, or nothing at all.
A document can be both, page by page. A contract typed in Word with a signed page scanned back in has text on nine pages and an image on the tenth — which is exactly the file that converts to Word with one page mysteriously blank.