Choosing a page size
A4 is the standard in most of the world, and Letter is the norm in the United States and Canada. If the PDF will be printed or submitted to an office, pick the size that office uses. Images are scaled to fit the page and centered without being stretched.
Fit to image makes each page exactly the size of its picture. This suits receipts, screenshots and signed forms you only plan to view on screen, because nothing gets cropped or padded with white space.
If your images have very different proportions, such as a square screenshot followed by a tall receipt, fit to image avoids awkward white bands. For a printed set, A4 or Letter with a small margin gives the most predictable result.
Remember that images are placed without distortion. A wide photo on a portrait page will have empty bands above and below, which is normal. If you would rather fill the page, crop the image to the page proportions first. A4 is about 1 to 1.41, so a 3 by 4 crop is close.
Margins and orientation
No margin lets a photo fill the page edge to edge, which looks good for pictures. A small margin leaves a thin white border that printers handle better, since many cannot print right to the edge. A large margin suits document scans where you want room for hole punching or binding.
Orientation is chosen automatically. A wide picture gets a landscape page and a tall one gets a portrait page, so you do not need to rotate anything yourself.
Keeping the PDF small enough to send
Phone photos are big. Ten pictures at 4 MB each make a 40 MB PDF that many email services refuse. Compress the images first with the image compressor, aiming for around 300 to 500 KB each for readable document scans, then build the PDF. You can also run the finished PDF through compress PDF.
For text documents, photograph in good light straight from above. Skewed, shadowed pages are hard to read and compress poorly.
What to know before you start
The tool accepts JPG and PNG files. If your photos are HEIC from an iPhone or WebP from the web, convert them to JPG first. Transparent areas in a PNG show as white on the page. The order of the list is the order of the pages, so check it before creating the file.
For a document scan, order matters more than anything. Number your photos in the gallery or rename them before adding them, and then check the list once more here. Rebuilding a 15 page PDF because page 4 and 5 swapped is annoying.