Images to PDF

Which page size

A4 is right when the PDF will be printed or filed with other documents. Sizing each page to its image is right for screenshots and scans, where white margins are just waste.

In Pillow, which is simpler than you expect

from PIL import Image

paths = ["one.jpg", "two.jpg", "three.jpg"]
pages = [Image.open(p).convert("RGB") for p in paths]

pages[0].save("out.pdf", save_all=True, append_images=pages[1:])

That is the whole thing. Pillow writes PDF but cannot read it, so this is a one way street. The convert("RGB") matters because an RGBA image fails here for the same reason it fails as JPEG.

Controlling the size on the page

pages[0].save("out.pdf", save_all=True, append_images=pages[1:], resolution=150.0)

The resolution decides how many pixels become an inch, so a 1500 pixel wide image at 150 is ten inches across. That is how you control the printed size without resampling anything.

A whole folder, in order

import pathlib
from PIL import Image

paths = sorted(pathlib.Path("scans").glob("*.jpg"))
pages = [Image.open(p).convert("RGB") for p in paths]
pages[0].save("scans.pdf", save_all=True, append_images=pages[1:], resolution=150.0)

sorted is doing real work there. Files named page1, page2, page10 sort into the wrong order, so zero pad the numbers when you create them.

See format support for what else Pillow writes.