That entirely depends on whether you're seeing performance that's acceptable to you. PDFs can often be a better option for text-heavy documents because they can usually be rendered at the correct resolution for printing at whatever size you need, whereas images have a fixed resolution and may sometimes look blurry on large-format prints. If you're dealing with a PDF that's just scanned from an image, though, you'll definitely be better off with a plain image.
As for whether to import directly into the layout, it depends on what kind of effect you're trying to achieve. Putting the PDF in the layout helps avoid the overhead of rendering the PDF while working in your plan view, which can make editing operations considerably smoother. However, if you're primarily interested in the background's alignment relative to objects in the plan (rather than just its alignment with the layout box), then it'll be easier to keep it aligned if you put it in the plan. If you do put it directly in the plan, it's possible to use layer sets to turn it off during regular editing while still having it visible in the layout.
