Building a PDF from photos. Order and orientation matter most: pages follow the order you arranged the files in, and phone shots often arrive rotated. The second issue is weight – a photo from a modern smartphone easily runs 5–8 MB, and ten such pages produce a PDF that no upload form will accept. If the file is going to be sent somewhere, compress the finished PDF. iPhone photos in HEIC are converted to JPG first.
The reverse task has two distinct answers that are frequently confused. Rendering turns each whole page into a PNG or JPG – you get a picture of exactly what appears on screen, text and layout included. Extraction pulls the embedded originals out of the PDF – the very files that were placed into it, without the surrounding text. If you want a 'screenshot of the page', use the first; if you want the source photographs, use the second.
On resolution: 96–150 DPI is plenty for screen reading and email. If the images are headed for text recognition, use at least 300 DPI – below that threshold OCR starts making mistakes on small type. For print, aim for 300 DPI and above.