Sheet flat, no flash, light from both sides, all four corners inside the frame.
Image to PDF, A4 portrait, images in reading order.
Shrink the file, then zoom to 100% and confirm the smallest text is still readable.
Shooting a sheet of paper from above with a ceiling light or flash gives a bright patch and a dark falloff, and auto white balance turns the page grey or yellow. Shoot from directly above with the sheet flat and the flash off, and put light on both sides if you can. Contrast cannot recover text that was shot inside a shadow.
Photos arrive at whatever resolution the phone produced, so a ten-page PDF ends up with ten different page sizes and the printer scales each one differently. Convert with A4 portrait selected so every page shares the same canvas, and crop to the paper edge first so the margins look even.
Scanned pages look white because the scanner binarises the image. A phone photo stays grey after conversion — the PDF container does nothing to the pixels. To get closer, raise brightness and contrast in a photo editor first and keep grayscale for invoices and IDs; hard black-and-white destroys handwriting and thin receipts. Then watch the size: one phone photo is 3-5 MB, so ten pages is 40 MB before you start, and compression brings each page down to a few hundred kilobytes because the redundancy is in the pixels.
Only if you threshold it in a photo editor. Grayscale is the safer target for receipts and IDs, because thresholding erases faint stamps.
Under about five degrees a crop fixes it. Beyond that a reshoot is faster than straightening.
Convert both, then merge and reorder. One page per photo is easier to fix later.