Page size, and when it matters
Fit each page to the image makes every page exactly as large as the image on it. Nothing is cropped, nothing is padded, and a document of mixed sizes keeps each one. This is what you want for screenshots, a photo album, or a set of scans you only intend to read on screen.
A4 or US Letter places each image centred on a standard page with a margin you choose. This is what you want when the file will be printed, emailed to an office, or uploaded to a form that expects a normal page size. Portrait images on a portrait page waste less paper; for mostly wide photos, choose landscape.
The margin only applies to the fixed sizes, because a page that fits its image has nowhere to put one.
Ordering
The order in the list is the order in the PDF. Files arrive in the order your browser hands them over, which is often not the order you see in your file manager, so check it before creating the document. The arrows move a page up or down, and the cross removes it.
A practical habit for scans: name the files so they sort correctly, and add them in one go.
Quality
JPEG and PNG are embedded directly into the PDF without re-encoding. A photo in the PDF is byte for byte the photo you dropped in, which means no quality is lost and also that the file is about as large as the sum of your images.
If the result is too heavy for email, resize the images first and then build the PDF. A 4000 pixel phone photo is far more detail than any printed page can show.
Questions
Which image formats work?
JPEG and PNG. These are the two formats the PDF format itself can embed directly, which is why the images keep their original quality instead of being re-encoded.
Why is my HEIC photo rejected?
Browsers cannot decode HEIC. Set your iPhone to Most Compatible under Settings, Camera, Formats, or convert the photos first.
Should I choose fit or A4?
Fit gives each page the exact dimensions of its image, which is right for screenshots and photo albums. A4 or Letter is right for anything that will be printed or attached to a form.
Are the images compressed again?
No. JPEG and PNG data is embedded as it is, so the PDF is roughly the size of the images combined.
Is there a limit?
Sixty images per document here, and beyond that your device memory. A hundred phone photos will produce a very large file.