Is it legally equivalent to a scanner copy?

That depends on who is receiving it. Many organisations accept any copy that is clearly legible and shows no sign of tampering. Some procedures still require the original document, or an officially certified copy. When something important is at stake, ask the recipient first.

The edges of the page are not detected. What now?

Usually the background is too close in tone to the paper, or a corner has fallen outside the frame. Put the sheet on something dark and re-shoot with all four corners visible. Failing that, drag the corners into place manually.

The text is not crisp — should I raise the resolution?

If the shot was blurred or out of focus, no. A higher resolution cannot create detail that was never captured. Re-shooting is faster. If the shot was in focus, the resolution you already have is plenty.

The finished PDF is too large.

Colour pages and a high page count add up quickly. For text-only documents, greyscale or black and white shrinks the result dramatically. If the colour has to stay, compress the images to a target size before bundling.

Turning a phone photo of a document into a proper scan

Three things separate a photo from a scan: perspective correction that squares up the sheet, background correction that removes uneven lighting and shadows so the paper reads white, and bundling the pages into a single PDF. Get all four corners in frame and avoid casting a shadow, and a tool can handle the rest.

Someone needs a signed contract as a PDF, or a receipt attached, or a certificate emailed over. There is no scanner in sight and all you have is a phone. Photographing it seems like it should be enough — and then the reply comes back asking for a proper scan.

What actually differs between a photo and a scan

It is not resolution. A phone camera out-resolves most flatbed scanners. Three other things differ.

Shape — the sheet in a scan is a clean rectangle; in a photo it is a trapezoid seen from an angle.
Brightness — a scan has an evenly white background, while a photo keeps the lighting gradient and the shadow of your own hand.
Format — what the recipient asked for is one PDF; what you have is several JPGs.

Those three are exactly what makes the file awkward on the other end. Slanted text is harder to read, shadowed areas print grey, and five loose photos have to be opened in the right order.

What the shot decides — and correction cannot undo

Correction refines information that is present; it cannot invent information that is missing. So a few things have to be right at capture time.

Get all four corners in frame. With a corner cut off there is no boundary to find, and nothing to square up against.
Shoot from directly above. The steeper the angle, the more the far edge is squeezed — and stretching it back out during correction makes it blurrier still.
Avoid casting a shadow. Anything between the light and the paper leaves one. Light arriving from the side, as through a window, is the easiest to work with.
Put the paper on a contrasting surface. White paper on a white desk has no findable edge; on something dark it is detected easily.
Check the focus. No amount of processing rescues a blurred shot.

How shadows come out — dividing, not subtracting

Raising the brightness uniformly lightens the shadow and the text with it. The method that works is different. First, estimate how the light fell across this particular photo. Lighting gradients and shadows are broad and smooth, so they survive almost unchanged when the image is scaled down; thin, sharp things like letters disappear at that scale. That difference is what separates the background from the content.

Divide the original by that estimated background and the bright areas divide by a bright background while the dark areas divide by a dark one, leaving the paper evenly white. Text is far darker than its local background, so it stays dark through the division. That is why letters in a shadowed corner come back.

Black and white, or keep the colour?

Mode	When to use it	Trade-off
Document	The default, and right for most paperwork	Whitens the paper but keeps colour — stamps, signatures and highlighter survive
Greyscale	Text-only documents headed for a mono printer	Drops colour, shrinks the file, and matches what the printer will produce
Black & white	Long text-only documents	Smallest files by far — but a red seal or faint pencil can vanish
Original	When the photo itself is the evidence	No correction at all — damage reports, accident scenes

Documents being submitted usually want the Document mode. A seal that disappears can put the validity of the paperwork in question.

Black and white forces every pixel to be either ink or paper, which is why the files get dramatically smaller — useful when emailing dozens of pages. The cost is everything in between: the colour of a seal, a faint pencil annotation. Keep it away from contracts and certificates.

Bundling into a PDF — choosing the page size

You have to decide on a page size. For something that will only ever be read on screen, matching the page to the image's own proportions avoids empty margins. For anything to be printed or filed with an institution, put it on A4 or Letter so the recipient's printer does not pick a scale of its own.

And check the order. Pages bundled out of sequence produce a document that reads as nonsense. A glance at the preview before you export is enough.

Keeping ID cards and contracts off other people's servers

Many scanning apps send your photo to a server to process it. The documents involved here, though, are usually ID cards, bank statements and contracts — exactly the ones that should not be sitting in someone else's storage. Switching does everything from perspective correction to PDF export inside the browser, so the file never leaves your device. Up to 20 pages per document.