Why do some adobe pdf documents allow cutting & pasting and some don't?

Like Korkstand said, it won't work for scanned images. Scanned images are more or less like jpeg photos with a pdf wrapper on them that allows acrobat to open them. You can OCR them and extract the text to Notepad or back into Word if that's your intent, but you'd basically need Photoshop to edit content in the pdf itself, and that would likely be sloppy looking.

If not scanned, you might have more luck, but there's a lot of ways to make a pdf, so it's still hard to say. If it's just a text document you might be better off re-typing, or seeing if you can convert to Word, then edit / fix formatting and change back to pdf.