JPG to PDF

Yes, you can combine both formats in one PDF.

No. The image data is embedded into the PDF rather than re-encoded, so the page looks exactly as good as the photo you started with. The quality ceiling is set by the original image — a blurry or heavily compressed JPG will look the same in the PDF. The conversion step itself does not add another round of compression on top.

Images become pages in the order they appear in the list before you convert, and you can rearrange them first. This is worth checking when you are combining photos of a multi-page document, because camera-roll order and selection order do not always match — particularly if the pages were photographed across more than one session.

For something that will only be read on screen, the equivalent of 150 DPI is plenty. For anything you might print, or that has small print like a receipt or a form, aim closer to 300 DPI. In practice a well-lit photo taken at your phone's full resolution and cropped tightly to the page beats a dedicated "document scan" mode that downsamples aggressively.

Not as HEIC files, which is what iOS saves by default — they have to be converted to JPG first. Setting the camera format to Most Compatible makes the phone save JPG from then on, which avoids the problem for everything you photograph afterwards.

Because matching the image size makes each page exactly as big as the photo on it, and a picture a few thousand pixels wide becomes a page far larger than a sheet of paper. Choose the A4 option instead if the document is going to be printed or sent to someone who will print it.