Deskew a PDF: straighten crooked scans automatically
A sheet that slides a few millimetres in the feeder, or a page photographed at a slight angle, ends up as a tilted scan. It looks careless, it prints crooked and it is harder to read — and OCR software makes more mistakes on slanted lines. This tool lets you deskew a PDF in your browser: it measures the tilt of every page, turns each crooked page back to level and leaves straight pages exactly as they were.
You see the detected angle on every page thumbnail, and a before-and-after preview with guide lines for the page you select. If the automatic angle is not quite right — for example on a page with a rotated stamp or a diagonal diagram — a slider lets you set the correction yourself to a tenth of a degree, or leave the page alone.
Your file is processed entirely on your device. Nothing is uploaded, which matters for contracts, ID copies and medical records.
How to straighten a scanned PDF
- Drop your PDF onto the upload area or click to choose it. Every page is measured and shown with its tilt, such as “Tilted +2.4°” or “Straight”.
- Tap a page to open the before/after preview. The guide lines make it easy to see whether the text is level.
- If needed, adjust the Correction for this page slider, nudge it by ±0.1°, or press Leave this page as it is.
- Choose the resolution for straightened pages: 200 DPI suits most documents, 300 DPI is best if you will run OCR or print.
- Press Straighten PDF, then Download PDF. Use Make it searchable to add a text layer again.
Deskew PDF features
Automatic angle per page
Each page gets its own measurement between −15° and +15° in 0.1° steps, so a batch of differently skewed scans is fixed in one pass.
Before/after preview
Compare the original and corrected page side by side, with optional horizontal guide lines.
Manual override
Set any angle by hand, nudge it in 0.1° steps, return to the detected value or skip a page entirely.
Straight pages untouched
Pages tilted by less than 0.2° are copied as they are, keeping their text, vectors and file size.
Your choice of quality
150, 200 or 300 DPI and three JPEG quality levels for straightened pages, at the original page size.
Private
Measuring, rotating and building the new PDF all happen in your browser.
How the skew angle is measured
Each page is rendered at about 100 DPI and turned into black and white with Otsu’s threshold. The tool keeps the bottom edge of every dark stroke — mostly the baselines of the text — and, for each candidate angle from −15° to +15° in 0.1° steps, projects those points onto a line at right angles to that direction. At the true angle the points of each text line stack into one narrow peak, so the profile becomes as jagged as it can get. The angle with the largest sum of squared differences between neighbouring bins wins, refined between steps by fitting a parabola. This is the classic projection-profile method used in document-imaging software.
Pages without clear lines of text, such as photographs, blank pages or a single diagram, are reported as “No text lines” and left unchanged unless you set an angle yourself.
What happens to straightened pages
Rotating a page by a fraction of a degree cannot be done with PDF page rotation, which only supports quarter turns. So each crooked page is drawn at the chosen resolution, rotated about its centre, and saved as an image at exactly the original page size, with the uncovered corners filled white. Any text layer on that page — for example from an earlier OCR — is lost, and links or form fields on it are not kept.
That is why pages that are already level are copied unchanged, and why the result offers a direct link to Make PDF Searchable: straighten first, then OCR, and the recognition will be more accurate. If the file gets large, Compress PDF can shrink it.
| Resolution | Typical A4 page size | Best for |
|---|---|---|
| 150 DPI | 1240 × 1754 px | Reading on screen, email |
| 200 DPI | 1654 × 2339 px | Everyday archiving (default) |
| 300 DPI | 2480 × 3508 px | OCR, printing, small print |
Tips
- Sideways or upside-down pages are a different problem — turn them with Rotate PDF first, then deskew.
- Photos of documents with a trapezoid shape need perspective correction rather than rotation; Scan to PDF does that from the original photos.
- Empty backs of a duplex scan can be cleared out with Remove Blank Pages.
- A page with rotated text boxes can fool any automatic method. Check the preview and correct it by hand.
Deskew PDF FAQ
How do I straighten a crooked scanned PDF?
Open it on this page. Each page’s tilt is measured automatically; check the previews, adjust any page if you like, press “Straighten PDF” and download the result. It is free, with no sign-up or watermark.
How accurate is the automatic deskew?
On pages with several lines of text the detected angle is usually within about 0.1–0.2° of the true tilt. Pages with very little text, rotated stamps or diagonal artwork can be measured wrongly — use the slider to fix those.
Will deskewing keep my text selectable?
Not on the pages that are straightened, because they are saved as images. Straight pages keep their text. Run Make PDF Searchable afterwards to add a selectable, searchable text layer to the whole file.
Why is the deskewed PDF larger than the original?
Straightened pages are stored as new images. Pick 150 DPI or Standard quality for a smaller file, or compress the result afterwards.
What angle range can it correct?
From −15° to +15°. Bigger tilts are rare in scans; for pages turned by 90° or 180° use Rotate PDF.
Is my document uploaded?
No. The PDF is read, measured, rotated and rebuilt in your browser, and the new file is created on your device.