How do I fix a crooked or skewed scan?
Use crop and perspective fix to drag the four corner handles onto the real corners of the page, and the scan is warped back into a true rectangle; use rotate for a page that simply went in sideways. PDF Scanner does both on the device. The limit worth knowing is that correction works on flat pages — it can square a sheet photographed at an angle, but it cannot flatten paper that is itself curved, which is why text near a book's spine stays distorted.
The short version
- Crop and perspective fix warps a page shot at an angle back into a rectangle by moving four corner handles.
- Rotate handles pages captured sideways or upside down.
- Perspective correction needs all four page corners inside the frame to work.
- Correction assumes a flat page — physical curvature, such as the gutter of a bound book, cannot be undone.
- Straightening before OCR measurably helps recognition, because text lines end up horizontal.
- All correction runs on the iPhone; nothing is uploaded.
Step by step
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Check what kind of crooked it is
A page rotated in the frame needs Rotate. A page that looks trapezoidal, wider at one end, needs perspective fix. A page whose text curves needs a rescan, because the paper itself was not flat.
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Open crop and perspective fix
The four corner handles appear over the detected outline. This is the same tool that trims the background away.
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Drag each handle onto a real page corner
Place them precisely on the paper's corners, not on a shadow or a table edge. Precision here is what decides whether the straightened result looks square.
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Apply and check the margins
After correction, look along the left and right margins. Parallel margins and horizontal text lines mean it worked.
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Rotate if the page is on its side
Rotate in ninety-degree steps until the text reads normally, then re-check the crop.
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Rescan if a corner was cut off
Correction has nothing to anchor to when a corner is outside the frame. A five second rescan beats fighting an unrecoverable capture.
Frequently asked
- Why can it not fix my scan?
- Almost always because a corner is missing from the frame, or because the paper was curved rather than flat. Perspective correction is a geometric transform on a four-cornered flat shape — take away a corner or add a curve and it has no valid solution.
- Does straightening reduce quality?
- Slightly, and mostly on the far side of a steeply angled shot, where fewer pixels covered that part of the page. A moderate angle straightens with no visible cost, which is why shooting off-axis to dodge glare is a reasonable trade.
- Should I straighten before or after running OCR?
- Before. Recognition expects horizontal text lines, so squaring the page first improves the result.
- How do I avoid crooked scans in the first place?
- Put the page on a contrasting surface, hold the phone parallel to it, get all four corners in the frame, and wait for the detected outline to settle before capturing.
- Can I fix a crooked scan without uploading it?
- Yes. Cropping, perspective fix and rotation all run on the iPhone in PDF Scanner, so the correction works offline and the page is not sent anywhere.
Last updated 2026-07-28