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Split a scanned book’s two-page spreads into single pages

You scanned a book. The scanner saw one sheet of paper, so that is what it saved: one page of the PDF holding two pages of the book, side by side. On a desk this is fine. On a tablet or an e-reader it is unreadable — every "page" is twice as wide as the screen, and zooming in means panning back and forth across a gutter for four hundred pages.

What you want is one book page per PDF page. Nothing in a normal PDF toolbox does that, and the reason is worth thirty seconds of your time, because it explains why every tool you have tried has refused to help.

Why every page tool refuses this

Page tools operate on pages, and to a PDF a page is one rectangle with one piece of content on it. "Split" means split the document — take a 200-page file and make two 100-page files. It has no concept of splitting a sheet down the middle, because the sheet is the atom it works in.

So when you search for "split PDF in half" you get tools that ask which page to cut after. That is the right answer to a different question. (If you are not sure which of the two you actually want, we wrote that one down too.)

The operation you need is crop, done twice, then interleaved. This page is the full version of that recipe — measuring the crop, the gutter, off-centre scans, right-to-left books, and the pass that finishes the job — so it is worth reading end to end before you touch a file.

The recipe, end to end

You are going to make two copies of the same document — one showing only the left half of every sheet, one showing only the right half — and then zip them together so they alternate.

  1. Crop the left half. Open the document in Crop PDF, and trim from the right by half the sheet's width. What remains is every left-hand page. Save it as left.pdf.
  2. Crop the right half. Go back to the original document, not the file you just made, and this time trim from the left by half the width. Save it as right.pdf.
  3. Interleave them. Put both into Alternate & Mix PDF, with left.pdf first. It takes page 1 from the left file, page 1 from the right file, page 2 from the left, and so on — which is exactly the reading order of the book.
  4. Check the order on the first four pages before you do anything else. If pages 2 and 3 are swapped throughout, your book reads right-to-left; see below.

Getting the crop values right

Measure, do not guess. Open the PDF and look at the page size in millimetres — our PDF inspector reads it out of the file without uploading anything, or any viewer will show it in document properties.

If the sheet is 420 mm wide (A3 landscape, which is two A4 pages side by side), then half is 210 mm. To keep the left page, trim 210 mm from the right. To keep the right page, trim 210 mm from the left.

Two things will make that slightly wrong, and both are easy to correct:

Crop the first two or three pages as a test before committing to four hundred.

Right-to-left books

Manga, and books in Arabic or Hebrew, read the other way: the right-hand page comes first. The recipe is identical except for step 3 — put right.pdf in first and left.pdf second. Everything else, including both crops, is the same.

What our tools will not do here

Two honest limitations, because they will cost you an hour if you find them out the hard way.

Split PDF in half will not do this. It cuts the document in half by page count, not each page in half by width — run it on a book scan and you get the first hundred double-wide sheets in one file and the last hundred in another, no closer to single pages than when you started. If you have not settled which of the two "split in half" jobs you actually want, that question has its own short answer.

Cropping hides, it does not delete. A PDF crop sets the visible window on the page; the far half is still in the file, just outside the frame. For reading on a tablet that is completely fine. But if you are publishing the result, or the far page contains something you would rather not ship, run the finished file through Flatten PDF or export it to images with PDF to JPG and rebuild — either of which discards what is outside the frame.

When a desktop tool is simply better

We would rather you finish the job than use our tools. If you are doing this regularly, or the scan is skewed enough that a straight vertical cut will not do:

The browser recipe above wins when you have one book, no software, and no wish to install any.

The finishing pass

Once the pages are single and in the right order, three things make the result pleasant to read:

  1. Straighten it. Book scans are almost never square. Deskew PDF rotates each page by the small angle needed to bring the text level.
  2. Trim the black edges. The dark band down one side is the scanner lid — that has its own short answer.
  3. Compress it. A book scan is hundreds of photographs and will be enormous. Compress PDF re-encodes the images; expect a large reduction on a colour scan and very little on a document that was already greyscale, which is normal and explainable.

Then open it on the device you actually read on, not on your desktop. Page size that looks fine on a 27-inch monitor is the whole question on a six-inch screen.

Tools this post uses: