Cropping PDFs for the iPad

The iPad version of iBooks got support for PDFs in a recent update. Overall support is quite good, however the iPad screen size is roughly half the size of an A4 or 8.5 by 11 page, so text can be very small. In many cases work formatted for the printed page includes huge margins, whether to accommodate binding or reformatting for different paper sizes, or just to facilitate holding a page without obscuring text. None of this is necessary on the iPad, so it’s often very useful to crop PDF pages in order to devote more of the screen to content.

I hope that Apple someday adds cropping as a core feature of iBooks, but for now the in-app zooming only works on individual pages: when you turn the page you are zoomed back out. To read a PDF without margins, you need to crop the PDF file itself.

I use the Skim application for the Mac to crop pages. The simplest approach is just to use the “Select Content” feature from the “Tools” menu, and then “Crop” from the same menu. This will eliminate the margins, however it won’t necessarily produce a PDF with the correct dimensions for the iPad screen: if your pages do not have exactly the right proportions then each page will be centered between gray borders.

To eliminate the borders, you need to crop to page sizes in the proportion 1:1.30729 (the iPad screen size, minus twenty pixels for the top status bar, is 1004×768). Further, you may want to consider the on-screen controls, which produce a 44-pixel header, a 44-pixel footer, and a page number which rises 81 pixels above the bottom of the screen. If you want your PDF to be readable even when the controls are visible, keep your content out of the top 4.38% and bottom 8.07% of the page.

In all cases, you need to tell Skim to export the file as a “PDF with embedded notes” in order for the application to re-encode the PDF and save your crop.

As an example, I wanted to format today’s McDonald v. Chicago Supreme Court decision for the iPad. While the default page size is 612×792, Skim’s “Select Content” feature produces a selection 320×536. This includes running headers, which I don’t mind being obscured by on-screen controls, but the content in this selection runs right to the bottom, which I’d prefer were still visible. I thus extend the crop box to a height of 583 (536∕0.9193). The resulting selection is narrower than iPad proportions, so I extend it 446 wide (583∕1.30729) by dragging out an extra 63 pixels on each side. The result is perfectly-sized pages with still-generous margins and text 36% larger than the original document. The converted PDF is available here.


I’ve also converted the Bilski v. Kappos decision, which may have more relevance to the tech crowd (although it doesn’t seem to clarify much about patent law).