Geometry
Crop a PDF
Academic papers and scanned books waste enormous margins, which is fine on paper and miserable on a tablet. Trimming them makes the text fill the screen without zooming.
Result
Cropping a PDF hides, it does not delete
Every PDF page has a media box, which is the sheet, and a crop box, which is the part a reader displays. Setting the crop box is what this tool does. It is instant, lossless and reversible, and text stays selectable because nothing is re-rendered.
The content outside the crop box is still in the file. It is not shown and not printed, but it can be recovered by resetting the box, and it still counts towards the file size. That makes cropping the wrong choice for removing sensitive material, and the right choice for improving readability.
The mirror option flips the left and right values on even-numbered pages, which is what you want for a scanned book where the inner margin alternates sides.
Common questions
How do I know how much to trim?
Start with the defaults, save, and look at the result. Most journal papers tolerate 15 to 20 mm all round. Scanned books vary wildly, so expect a second attempt.
Will it make the file smaller?
No. The hidden content is still stored. If size is the goal, use compress PDF instead.
Can I undo a crop?
Your original is untouched, so keep it. Within a cropped file, most desktop PDF editors can reset the crop box to the media box.
Other PDF tools