Most documents waste a surprising amount of the page. A scan with a generous border, a slide deck exported with padding, an academic paper built for print margins you don't need on screen — all of them surround the actual content with white space that's doing nothing but shrinking the readable area.
Cropping fixes it, but doing it by hand means measuring margins and typing numbers. Auto Crop skips that: it finds where your content actually starts and stops, and trims to it.
What Auto Crop actually does
It detects the content bounds on the page — the edges of the real ink — and sets the crop to hug them, discarding the empty margin around it.
The important detail is what happens across a multi-page document: Auto Crop detects the bounds for each page individually. That matters enormously for scans, where every page has slightly different margins because the paper sat a few millimetres differently on the glass each time. A single fixed crop applied to all of them would clip content on one page and leave white space on another. Per-page detection handles each one on its own terms.
The steps
Open Blackpdf's Crop PDF tool and drop your file in.
Under Quick Actions, click Auto Crop — it detects the content on the page and sets the crop box to fit.
Choose what it applies to under Apply To:
- All pages — each page gets its own content-aware crop.
- Current page — just the one you're looking at.
- Custom — a range like
1-3, 5, 7-10.
Check the preview. Auto Crop is a detection, not a guarantee — look at a couple of pages before committing, especially if the document has faint content near the edges.
Click Crop PDF and download.
When to crop manually instead: if you want a consistent crop across every page (so all pages end up the same size), set the top, bottom, left, and right margins by hand rather than using Auto Crop — per-page detection deliberately gives each page its own boundaries, which can leave pages at slightly different sizes.
Common questions
Does cropping delete the content outside the crop?
No — and this surprises people. Cropping changes the visible area (the CropBox); the content outside it is hidden, not destroyed. That means the file usually doesn't get smaller, and someone with the right tool could reveal it. If you need content genuinely gone, that's redaction, not cropping. The page boxes explainer covers the mechanics.
Auto Crop cut off part of my content.
It detects ink, so very faint content near the edge — a light footer, a pale watermark, a page number in grey — can fall below the detection threshold. Check the preview, and if something's clipped, set the margins manually with a little breathing room instead.
My pages ended up different sizes.
That's expected with Auto Crop on a multi-page document: each page is cropped to its own content, so pages with more content stay larger. If you need uniform pages, either crop manually with fixed margins, or run the result through Resize PDF Pages to normalise them.
Will it work on a scanned PDF?
Yes — scans are the ideal case, because their margins vary page to page and per-page detection handles exactly that. If the scan is also grey or shadowed, enhance it first so the content bounds are cleaner to detect.
Does cropping reduce the file size?
Barely. The hidden content is still in the file. If size is the goal, compress it instead.
Wrap-up
- Drop your PDF into Crop PDF.
- Click Auto Crop under Quick Actions.
- Pick All pages, Current page, or a Custom range.
- Check the preview, then Crop PDF.
Auto Crop's per-page detection is what makes it good on scans — every page gets trimmed on its own terms. If you'd rather every page end up identically sized, set the margins manually. For the full walkthrough of all three crop methods, see the main Crop PDF guide.