Deskew PDF
Automatically straighten crooked scanned pages — with a preview of every page and no loss of scan quality.
Drop your PDF hereChoose a file to get started
Free tier supports files up to 50MB.
Processed in your browser — files never leave your device. How it works
How to Straighten a Crooked Scanned PDF
Feed a stack of paper through a scanner, or snap pages with a phone, and some of them will come out slightly crooked. A tilt of two or three degrees doesn't sound like much, but it makes a document look careless, is tiring to read, prints with uneven margins and lowers the accuracy of text recognition (OCR). PDFDoco measures the tilt of every page, shows you the angle with a before-and-after preview, and straightens the crooked ones — without re-compressing the scan, so it stays exactly as sharp as the original. Everything runs in your browser and the file is never uploaded.
- 1
Upload your scanned PDF
Every page is analysed in your browser and its tilt is measured from the lines of text.
- 2
Review the detected angles
Each page shows its angle with a before/after preview. Crooked pages are selected for you.
- 3
Adjust if you like
Fine-tune any angle in 0.1° steps, or untick a page to leave it as it is.
- 4
Download the straight PDF
Pages are rotated without re-compressing the scan, so quality is exactly the same.
No quality loss
The scan image is rotated as-is with a PDF transform — never re-rendered or re-compressed.
You stay in control
See every page's angle, compare before and after, adjust in 0.1° steps or skip pages.
Private scanning
Detection and correction run on your device. Your scans are never uploaded.
How automatic deskew works
Lines of text are the most reliable clue to a page's angle. Each page is rendered at low resolution and turned into black and white, then the dark pixels are added up along lines at angles from −10° to +10°. At the right angle the rows of text stack into sharp peaks with clean gaps between them; at any other angle they blur together. The tool searches in 0.5° steps, then refines to a fraction of a degree, and reports how confident it is. Pages without text lines — photos, blank pages — are flagged instead of being guessed.
Straightening without losing quality
The usual way to deskew is to rotate the image pixels and save a new image, which softens text and adds compression artefacts every time. Here, the original scan stays inside the PDF untouched and the page is told to display it at the corrected angle — the same kind of instruction PDF viewers use for zoom. The result looks identical to a re-rendered page but keeps every pixel of the original, and the file size barely changes. Any hidden OCR text layer rotates with the image, so search and copy keep lining up with the words.
Getting the best results
- Text-heavy pages give the most accurate angles; a few lines are enough.
- If a page is turned 90° or upside down, fix it first with Rotate PDF.
- Deskew before running OCR — recognition is noticeably better on level text.
- For very large scans, straighten first and then shrink the file with Compress PDF.
Fit, fill or enlarge?
Turning a rectangle always pushes its corners outside the original outline, so something has to give. Fit keeps the page size and shrinks the content a few percent, ideal for printing on standard paper. Fill keeps the page size and enlarges slightly so no white wedges show, at the cost of a thin strip at the edges — usually just scanner margin. Enlarge page keeps the content at exactly 100% and makes the page a little larger, which suits archiving and measured drawings.
Pro tip: scanned a book with facing pages at different angles? Each page is measured separately, so a stack of mixed tilts is straightened in one go.
Frequently Asked Questions
What does deskew mean?
Deskewing means straightening a page that was scanned or photographed at a slight angle. The tool measures how many degrees the lines of text are tilted and rotates the page by the opposite amount so the text runs perfectly horizontal again.
How does the tool detect the angle?
Each page is rendered small, turned into black and white, and the dark pixels are projected at many angles between −10° and +10°. When the angle matches the text lines, the rows of text line up into sharp peaks, so the sharpest projection gives the tilt — first in 0.5° steps, then refined to a few hundredths of a degree.
Will straightening reduce the quality of my scan?
No. Most deskew tools re-render each page as a new image, which softens text and changes the file size. PDFDoco rotates the existing page content with a PDF transformation instead, so the original scan image is kept byte for byte and only displayed at the corrected angle.
Will the corners of the page be cut off?
Not with the default "Fit" setting: the rotated content is scaled down very slightly (about 4% for a 2° tilt) so all of it stays on the page, and the uncovered corners are filled with white. "Fill" scales up instead so no white corners appear, trimming a sliver of the edges, and "Enlarge page" keeps the content at 100% and makes the page a little bigger.
What happens to pages that are already straight or not scanned?
Pages tilted less than the threshold you choose (0.3° by default) are left untouched. Pages that contain no images — normal digital documents — are recognised as not scanned and skipped automatically. You can still set an angle on any page by hand.
Does it work with photos of documents or handwriting?
It works best on pages with lines of printed text. Handwriting, tables and forms usually work too. Pages that are mostly a photo or picture have no text lines to measure, so they are marked as unsure and not changed unless you set the angle yourself.
Can it fix pages that are sideways or upside down?
Deskew corrects small tilts up to 10°. For pages that are turned 90° or 180°, use Rotate PDF first, then deskew if they are also slightly crooked.