Sort them in the album first so the conversion order is already right.
Choose A4 with the aspect ratio preserved to avoid distortion.
Shrink the finished PDF rather than starting from lower-resolution images.
The camera roll sorts by capture time, which is rarely the order you want for a set of documents. Collect the images into a dedicated album first, or reorder them in the file list before converting — far cheaper than discovering the mess afterwards.
Photos are typically 4:3 or 16:9; A4 is 1:1.414. Forcing an image to fill the page stretches it. Keep the aspect ratio, centre it and leave white margins. The image-to-PDF tool here preserves proportions by default and never crops.
Phone photos run 3–5MB each, so ten pages can be tens of megabytes. For archiving, compressing afterwards usually cuts size by two thirds with no visible difference. For ID documents you may prefer to keep the original sharpness.
JPG and PNG are. Export HEIC to JPG from the Photos app first.
One per page by default, which is what documents and receipts usually need.
Use the up/down controls in the file list before converting.