It would depend on a few things.
What are you using to make the PDFs? Are they mostly text or images?
For smaller file size you want to keep as much text as possible represented as text in the PDF (as opposed to as an image or outlined vectors).
For images you usually want to both optimize the format (saving as jpgs is best for photos, gifs or pngs for simpler art) and make sure you’re saving them close to the dimensions they’ll appear in the final document before putting them into the PDF.
An example might be taking an image directly from you camera, dropping into your document, then converting to a PDF. You should resize the image first so it’s closer to your final dimension and a smaller file.
There are also quality settings when making the PDF, but usually if you handle the source files right you don’t have to mess with those.