Pages can not import a PDF.
Provided that the PDF has not been locked to prevent copying, you can select by dragging, then Edit > Copy (command c). In Pages Edit > Paste (command v).
It's easy to print a Pages document on paper, but that doesn't mean that it has to be easy to convert a paper document to Pages. Pdf is an electronic equivalent to a paper printout; going from Pages to pdf throws out a lot of information that is unnecessary to the "printout." Ther'e no easy way to get that information back if you want to convert the other direction.
That may not matter for what you are trying to accomplish. Your pdf scan is essentially an image, which you can use as a background for a Page Layout document. You can then put a text box over each field where you want to type. Obviously, this is more practical if the form doesn't involve too many fields.