The main reason is likely file format, not just camera vs. scanner quality. PNG uses lossless compression, which preserves all image data but often creates larger files for photographed documents. Many PDFs store images internally as JPEG, which is lossy-compressed and usually much smaller at similar visible quality.

So if your goal is a readable document with a smaller file size, saving the photo as JPEG instead of PNG will usually help a lot. A scan can also look more efficient because the document is flat, evenly lit, and easier to compress.

A practical workflow is:

• crop tightly to the document
• rotate/perspective-correct if needed
• resize only as much as necessary for readability
• save as JPEG at moderate quality rather than PNG

If you need a PDF, you can place the JPEG into a PDF afterward. For document images, that often gives a good balance of readability and file size.

Use PNG only if you specifically need lossless output; otherwise JPEG is typically the better choice for this kind of document capture.