Estimate scan file sizes by resolution.
Scanners and cameras are specified in incompatible units, and converting between them settles the question of whether a phone can replace a scanner. An A4 page at 600 dpi is about 34 megapixels, so a 12-megapixel camera filling the frame delivers roughly 350 effective dpi — enough for legible text and OCR, well short of archival scanning. File sizes assume a ten-to-one JPEG ratio for both.
Scan resolution
Scan megapixels = width inches x dpi x height inches x dpi / 1,000,000
Camera equivalent
Effective dpi = square root(camera pixels / page area in square inches)
For everyday documents and receipts, yes — 300 to 400 effective dpi is enough for reading and OCR. For photographs, fine print or archival work a flatbed still wins on resolution, flatness and even lighting.
Perspective distortion, uneven lighting and lens softness at the edges all cost real quality that pixel count does not capture. Document scanning modes correct geometry and contrast, which closes most of the gap.