Phone photos of receipts, IDs, homework and signed pages are JPGs; the portal you need to submit them to wants one PDF. A PDF also prints at a predictable size and keeps pages in order. The JPG data can be placed in a PDF as it is, so this conversion does not need to recompress the picture.
Add several files and you can also combine them into a single PDF. The defaults suit most files. Under Options you can choose the page size.
Each picture becomes one page. The page can match the picture's own size or be A4 or US Letter; on paper the sheet turns to suit the picture, which is shrunk to fit inside the margins and never enlarged.