Merge the fronts and backs of a scan

Why the scanner leaves you the backs in reverse

Nearly every home all-in-one and many office scanners have an automatic feeder, but they read only one side of the sheet. To get the backs too you always do the same thing: you feed the pile through, turn it over, and feed it through again. Two PDFs come out, and the second has a flaw that is not your fault: when you turn the pile over, the last sheet ends up on top and gets read first.

With five sheets the first file holds fronts 1, 2, 3, 4 and 5, the second the backs 5, 4, 3, 2 and 1. The document you want is something else again: front 1, back 1, front 2, back 2, and so on. This page does exactly that sum: it takes the pages from the two files, reverses the order of the backs and interleaves them. If your scanner already saves the backs in the right order, just untick «The backs are in reverse».

By hand, twenty sheets mean some twenty drags in Organize PDF, and one mistake halfway puts every following back under the wrong front.

Two files, or one single file with everything in it

You can load two files, one per box, or both in the first box, together or one after the other. The page works out which is which from the name, with words like fronte, front, recto or anverso against retro, back, verso or dorso. If the name says nothing, a file loaded after the other counts as the backs, and for two files chosen together the name order decides, read with numbers as numbers: scan 9 before scan 10, and Scan before Scan 1, the name a computer gives a copy so as not to overwrite.

When the choice does not come from the name, the page says so, and below there is the button to swap the two files: guessing in silence would give a document with every back before its own front.

Some scanners instead save everything in one single file: first the fronts, then the backs. You load it in the first box and press «It is one single file, fronts and backs»: from the file alone the page cannot tell it from a file of fronts waiting for its backs. It takes the first half as fronts, one page more if they are odd, which is the sheet whose blank back got skipped. If the dividing line is elsewhere, you type it in «How many pages are fronts».

When the numbers do not add up

The most common case is one front too many: ten fronts and nine backs. It happens when the last sheet has a blank back and the scanner, with the option that skips empty pages, did not even save it. With the backs in reverse that gap sits at the head of the backs file, and whoever pairs «the first with the first» shifts every pair by one place. Here the sheet without a back stays on its own, at the end, and the page says so.

If instead the sheet without a back is the first one, for example a cover, you choose it in «The pages without a partner go» and the pairing moves by one place.

But the same option skips a blank back anywhere in the pile, and counting cannot tell where. Looking can: in the preview the sheet without a back is the first one whose front and back do not match. Press «No back» on that sheet and the backs after it move on by one sheet; if two or more are missing, you do it on each. If instead the backs are more than the fronts, nearly always the two files were loaded the wrong way round, and one tap on «Swap fronts and backs» is enough.

Upside-down backs: it depends on how you turn the pile

To feed the pile through again you can turn it in two ways. If you turn it like the page of a book, on the long edge, the backs come out upright. If you turn it from top to bottom, on the short edge, they come out upside down, and the order is reversed all the same: every other page is upside down.

That is why there is the choice «Turn the backs by 180°». The rotation is added to the one the page already had: a sheet scanned in landscape stays landscape and only gets turned upside down, instead of being stood upright by mistake. In the preview the backs already show turned, so you check before downloading. If only a few pages are crooked, and not all the backs, the right place is Rotate PDF, where you turn them one at a time.

Look at the pairs before downloading

Below the button there is the preview, sheet by sheet: the front and next to it its back, with a note of which file and which page each one comes from. It is the check that matters, because the PDF that comes out is right only if the front of sheet 3 really sits next to the back of sheet 3. Usually a glance at a printed page number is enough. The thumbnails get drawn as you scroll, so even a hundred sheets open at once.

If a sheet has its back before its front, «Swap» on that sheet fixes it without touching the others; if everything is wrong, usually it is enough to untick the backs in reverse, or to swap the two files.

The pages are copied as they are, without being turned into images: if the PDF had text inside, the text stays selectable and searchable, and the file weighs about as much as the two starting ones together. If instead it is a scan with no text, after putting it in order you can run it through Make a scanned PDF searchable, which puts the recognised text underneath.

What it does not do

It does not invent pages: where a back is missing no blank page appears, and where the numbers do not add up the page says so instead of filling the gap. It does not remove blank pages: if the scanner saved them, they stay where they are, and they come out afterwards with Organize PDF.

It does not merge more than two files: to put several documents in a row, one after the other, there is Merge PDF. And it does not rewrite PDFs protected by a password or by restrictions: it tells you as soon as you load them and sends you to Remove a PDF password, because saving anyway would mean handing you a file that then does not open.

It does not carry over the bookmarks: they would point to pages that have changed place. The title and the author do pass, and they come from the file of the fronts.