Does Preview compress PDFs when saving?

Hi. My scanner can group separate scans into a single PDF on its own, but it is significantly slower than scanning documents separately and merging them with Preview.

Since scans are essentially saved twice when going the "merge with Preview" route (once when scanned, again when merged), I'm curious to know whether Preview compresses PDFs with each save. If so, I'd like to know how lossy this process is. Anyone know?

Thanks in advance for any help / info!