Crop a PDF online, automatically or to the millimeter

Crop a PDF online: the full process

Cropping a PDF means redefining the visible part of each page: you remove the wide white margins of an article, the dark bands of a scan, or bring a document to a specific paper size. Certyneo Reader's “Crop” tool does all of this in your browser, on a computer, tablet or phone, with no account and without sending the file. Here is the full process, from the original document to the cropped PDF.

1. Open your document in the reader at the top of this page, by drag and drop or from your files. If it is password-protected, enter the password: it is checked on your device, not on a server.
2. Choose the mode. “Trim margins” removes a band along the edges of the page; “Change the paper size” places the content on an A3, A4, A5, US Letter or US Legal page.
3. Set the crop: click “Detect white margins” for automatic cropping, or enter the values in millimeters, side by side.
4. Specify the scope: the whole document, or only some pages whose numbers you type. The preview shows the result on the first page affected.
5. Apply, then scroll through the document. If a page is not right, the Undo button restores the previous state as long as you have not saved. Finally, download the cropped PDF.

The crop is computed on your device and Certyneo never receives the file. The tool is free, adds no watermark and imposes no limit on the number of uses; only the document size is capped, at 150 MB. It is part of the free online PDF reader, which brings all the tools together on the same document.

Crop a PDF or change its paper size: what is the difference?

The tool's two modes meet two distinct needs. The first removes area from the page: the page gets smaller, and the content keeps its size. The second changes the page's dimensions to match a standard paper size: the content is then enlarged or reduced to fit.

In practice, trim the margins when you want to gain room on screen, and change the paper size when the document has to go through a printer or join other pages with standard dimensions.

How to crop the white margins of a PDF automatically

The “Detect white margins” button analyzes each page and crops it as close as possible to its own content. It is the fastest method for a document whose margins vary from page to page: a slightly skewed scan, an article where some pages have a full-width image, a thesis with airier title pages. Each page gets its own frame, calculated from what it actually contains.

Trim a PDF's margins to the millimeter

When you want an identical result on every page, for example for a book or a report with a regular layout, enter the margins to remove at the top, bottom, left and right, in millimeters. The “Same margin on all four sides” option copies a single value to all four edges. The pages then all keep the same size, which makes for more consistent reading.

What if the detection cuts too much or too little?

The detection looks for anything that is not white. An isolated page number at the very bottom, a stamp in a corner or a very thin header line count as content and limit the crop on that side. Conversely, a grayish or stained scan may be seen as content across its whole surface. In those cases, switch to the millimeter setting, check the preview, and use Undo to start again if the result is not right.

The black bands that often surround a scanned page are removed the same way, in millimeters, since they are not white. If the scan then needs to become searchable, crop first, then run it through text recognition to make a scan searchable.

Change a PDF's size to A4, A5 or US Letter

The “Change the paper size” mode places each page on a new page with standard dimensions. The content is enlarged or reduced without distortion, that is, keeping its proportions, then centered. Annotations, such as highlights or comments, follow the content and stay in place.

For orientation, there are three choices: keep each page's own, make everything portrait, or make everything landscape. Keeping each page's orientation is the safest option for a document that mixes portrait text and landscape tables.

The most frequent case is a PDF received in US Letter size that has to be printed on A4. The two sizes are close but not identical: a Letter page is slightly wider and shorter. Once switched to A4, the page prints with no clipped edge and no scaling imposed by the printer driver. The same mode is used to harmonize documents of different sizes before you merge them into a single PDF, to get a file with uniform pages.

Cropping a PDF erases nothing: what happens to content outside the frame

One point is essential and often misunderstood: cropping hides; it does not erase. Technically, the tool changes the page's visible area, which the PDF format calls the “CropBox”. Everything outside the new frame stays in the file; it is simply no longer displayed or printed.

This has two consequences. The good one: cropping is reversible, since nothing is lost. PDF software that lets you edit the visible area can bring the original margins back. The less good one: cropping does nothing to protect confidential information. If you crop a page to hide an account number, a name or an address, that data remains readable to anyone who knows where to look.

To truly make information disappear, use the tool to permanently erase a sensitive passage from a PDF. Redaction removes the content from the file instead of hiding it. You can then crop the page if you also want to reduce its margins.

Another logical consequence: since the content outside the frame is kept, a cropped PDF is no lighter than the original. If the file size is a problem for sending, then run it through PDF compression.

Crop only some pages, including rotated pages

By default, the crop applies to the whole document. Choose “Only some pages” to limit the operation: the other pages keep their original dimensions. Page numbers are entered using a flexible syntax shared by all the reader's tools.

• “5” designates a single page.
• “1-3” designates a range, from page 1 to page 3.
• “8-” runs from page 8 to the end of the document.
• “-3” runs from the start of the document to page 3.
• Several items are separated by commas or semicolons, and duplicates are merged.

Cropping works on the page as it is displayed, rotation included. If a page has been turned a quarter turn, the “top” margin you enter is the one you see at the top of the screen, not that of the original page. To avoid any confusion, first turn the pages the right way with the tool to rotate a PDF and save the rotation, then crop.

For a badly ordered document, an appendix to move or an empty page to remove, go through rearranging pages with thumbnails before cropping: you will then crop pages that are already in place.

Why crop a PDF: four common situations

Cropping is a small step that noticeably improves reading or printing comfort. Here are the cases where it helps most.

Read an article or a scan on an e-reader or tablet

A scientific article or a book scan often keeps wide margins designed for paper. On a tablet or e-reader screen, they reduce the size of the displayed text. Once they are removed, the text fills the whole screen and can be read without zooming.

Clean up a scanned document

A scanner or a photo of a document often leaves dark edges, a binding shadow or a tabletop around the page. A crop in millimeters keeps them out of the display and the printout.

Print a foreign document on A4

A contract, a form or instructions received in US Letter or US Legal size print poorly on A4 paper. Switching to A4 settles the matter before printing.

Prepare a consistent file

Before assembling documents from different sources, bring them to the same size. The final file reads and prints evenly, and you can then add page numbers to the assembled file and send it for electronic signature without switching tools. There is also a free desktop version offering the same tools offline: see the page about the free PDF reader for Windows 11.

More questions about cropping a PDF

Can I crop a PDF on a phone?

Yes. The tool works in recent browsers on Android, iPhone and iPad, just as on a computer. Entering millimeters works the same; automatic margin detection saves you from typing values on a small screen.

Does cropping affect the quality of the text or images?

No. Cropping changes the displayed area, not the content: the text stays sharp and selectable, and images keep their resolution. In paper size mode, the content is only scaled, without distortion.

How do I go back after cropping?

As long as the PDF has not been saved, the Undo button restores the previous state. After saving, the content outside the frame is still present in the file; nevertheless, keep a copy of the original document if you plan further adjustments.

Can I crop a password-protected PDF?

Yes, provided you know the password. Enter it when opening the file: it is checked on your device. The document then opens normally and all the tools, including cropping, are available.