The portal rejected my scan but did not say why. Where do I look first?

At the resolution before anything else. Portals that state a density check it mechanically, and the message they return is usually generic. Open the file, read what the panel says the field contains, and compare it against the specification — that check costs nothing and resolves a large share of these rejections.

A form wants a signature at 300 dpi. How large should the paper area be?

Whatever the form states, and if it states nothing, scan the signature box as drawn. A signature strip of 4 by 1.5 inches at 300 dpi is 1200 by 450 pixels. Scanning a small mark at very high density does not help; the ink boundary is not that sharp on paper.

My scanner offers 600 dpi optical and 9600 dpi interpolated. Which figure is real?

The optical one. The interpolated figure is the scanner running the same operation this site runs, in its own firmware, and it adds no detail for the same reason. Scan at the optical figure or below it.

The scan is correct but the file is described as too large to upload. What now?

That is a different requirement from resolution and it is answered by re-encoding, not by rescaling — reducing the pixel count to satisfy a transfer limit will usually break the density requirement you just met. Keep the grid, keep the field, and lower the JPEG quality instead.

Preparing a scan to a specified resolution

A submission is rejected for resolution far more often than for anything else, and the fix is usually a header edit rather than a new scan. Here is the order to do it in.

Why resolution is the usual culprit

An automated intake has very little to check. It can read the pixel dimensions, it can read the density field, and it can read the file’s length. Of those three, the density field is the one most likely to be wrong through no fault of the person submitting, because scanner drivers, phone cameras and image editors all write it according to their own defaults and none of them knows what the form wants.

The consequence is a scan that looks perfect, satisfies the pixel requirement comfortably, and is refused because a header says 72. Nothing is wrong with the picture. Something is wrong with a number attached to it.

The procedure, in order

01Read the requirement as two numbers, not one. A specification that says 300 dpi is incomplete on its own. Find the physical size it applies to — the paper, the photograph, the signature box — because density and size together are what fix the pixel count.
02Multiply before you scan. Inches multiplied by the stated density gives the pixel count you need. A 35 by 45 mm photograph at 300 dpi is 413 by 531 pixels; an A4 page at 300 dpi is 2480 by 3508.
03Set the scanner to the same density, not higher. Scanning at 1200 dpi and reducing later spends time and produces a larger file for no gain past the point where the optics stop resolving. Match the requirement and let the scanner write the field itself.
04Open the file here and read what it claims. The panel reports the density found in the file and where it was stored. If the field is missing, it says so and treats 72 as an assumption rather than a fact.
05Correct the field without touching the image. Leave the operation on print size only, set the resolution to the figure required, and export. The scan data is copied byte for byte and the header now states the right number.
06Check the pixel count against the size one last time. The exported file must satisfy both halves: the field must read as specified, and the pixel count divided by the physical size must reach the same figure. The panel prints both so the comparison takes a second.

The counts most often asked for

Pixel counts required at 300 dpi
Subject	Physical size	Pixels
Passport-style photograph	35 x 45 mm	413 x 531
Signature strip	4 x 1.5 in	1200 x 450
Document page, A4	210 x 297 mm	2480 x 3508
Document page, Letter	8.5 x 11 in	2550 x 3300
Certificate, A5	148 x 210 mm	1748 x 2480

Where a portal specifies a pixel width and height instead of a density, it is asking a different question and this is not the instrument for it — set the two numbers with a tool built to take them directly, and come back here only if a density is also stated.

What can still go wrong afterwards

Two failures survive a correct header. The first is a scan whose pixel count genuinely does not reach the requirement, which the panel will show as an achievable resolution below the one specified; that needs a re-scan, not an edit. The second is a portal that measures the effective resolution at the size it intends to place the image, which can differ from the stored figure — in that case the pixel count is the number to argue with, and it is printed on the panel next to everything else.

Questions from the queue

The portal rejected my scan but did not say why. Where do I look first?: At the resolution before anything else. Portals that state a density check it mechanically, and the message they return is usually generic. Open the file, read what the panel says the field contains, and compare it against the specification — that check costs nothing and resolves a large share of these rejections.
A form wants a signature at 300 dpi. How large should the paper area be?: Whatever the form states, and if it states nothing, scan the signature box as drawn. A signature strip of 4 by 1.5 inches at 300 dpi is 1200 by 450 pixels. Scanning a small mark at very high density does not help; the ink boundary is not that sharp on paper.
My scanner offers 600 dpi optical and 9600 dpi interpolated. Which figure is real?: The optical one. The interpolated figure is the scanner running the same operation this site runs, in its own firmware, and it adds no detail for the same reason. Scan at the optical figure or below it.
The scan is correct but the file is described as too large to upload. What now?: That is a different requirement from resolution and it is answered by re-encoding, not by rescaling — reducing the pixel count to satisfy a transfer limit will usually break the density requirement you just met. Keep the grid, keep the field, and lower the JPEG quality instead.