Optimize PDF · guide
What Auto crop PDF does
Auto crop PDF trims the empty margins off every page for you. It measures where the content actually is on each page, including inside scanned images, then crops every page of the same size to one shared box with about 2 mm of space around the content, so a scanned book or a wide-margined paper reads larger on screen. Nothing on the page is re-encoded, and the file stays in your browser.
How to auto crop PDF in your browser
- Choose a PDF with wide margins Select the PDF to trim, such as a scanned book or a paper with generous margins. It opens in this tab and is not uploaded.
- Trim the margins Select Trim the margins. Each page is measured in the browser to find where its text and images end, even when the whole page is a scanned image.
- Check and save Look through the result, especially pages with page numbers or notes near the edge, then save the new PDF. The result reports how much came off each side and how precise the measurement was.
Worked example
Trim large white margins from a scanned booklet.
- Starting point
- Choose the scan and inspect faint notes near its edges. A scanned page is usually one full-page image, so its white margin sits inside the picture, where only a rendering of the page can find it.
- What to do
- Select Trim the margins. Each page is rendered in this tab at 100 DPI (coarser for very large pages) and scanned inward from all four sides, and every page of the same size is cropped to one box that fits them all, plus 6 points (about 2 mm). Then inspect the saved pages.
- Result
- Expect the same scans, unchanged, on smaller pages: only the page boxes change, not the images. The result gives the share of page area removed and counts any pages it left alone.
If the result is not right
If margins remain large, inspect edge marks and try the separate Crop tool for a manual boundary. Check faint notes and pages with different sizes before relying on the result.
Limits worth knowing
- Dust and isolated marks can affect ink detection. Cropping hides outside content; it does not delete that content.
Questions and answers
Why are all my pages cropped to the same size?
By design: the crop is the smallest box that fits the content of every page of that size, so pages stay uniform and the text does not jump about as you scroll. Pages of a different size, such as a landscape fold-out, get a box of their own. The cost is that one page with a wide table keeps that margin open on all of them; Crop PDF lets you draw a tighter box yourself.
Does auto crop delete anything from the PDF?
No. It makes the pages smaller and hides the margins; it does not remove what was in them. The content that remains is byte for byte what it was, and anything outside the crop is still in the file and comes back if the page is enlarged again. Use Redact PDF if something must not stay readable.
Why didn't it trim as much as I expected?
Something near the edge counts as content: a page number, a running header, punch holes, a scanner shadow or a line of specks down one side. A mark about 1 mm across or larger holds the crop open, while a single dust speck does not. A dark scanner edge down one side, wider than about 1 mm, counts as content too: nothing comes off that side, the top or the bottom, and only the opposite margin is trimmed. Where dark edges cover more than half of the border, they are read as the background instead, so they are cut back to about 2 mm and the paper's own margins stay. A page whose border has no single main colour, such as a photo that runs off the page, is left uncropped, and the result says how many were. Crop PDF lets you draw the box yourself.
Does it work on scans with grey, beige or coloured backgrounds?
Yes. The background is measured from each page's own border rather than assumed to be white, with more tolerance on a noisy scan, so off-white photocopies and slides with a plain coloured background can be trimmed. The limit is a flat strip of colour along the edge of a picture, which looks the same as a margin and can be trimmed with it.
Will the cropped pages look bigger on a phone, e-reader or printout?
On screen, yes: a viewer that fits the page to the width of the window now fits the content instead of the margins, which is the point for reading a scanned book on a small screen. The pages themselves are smaller, so a printer set to fit the page to the paper enlarges them, while printing at actual size puts the smaller page on the same sheet. The space kept around the content is fixed at about 2 mm.