Merge odd and even pages from 2 scan passes into 1 PDF in reading order

Why a plain merge gets it wrong

A single-sided feeder scans 1 side per pass. You scan the stack once for the fronts, turn it over and scan it again for the backs, and end up with 2 files. Merging them puts every front first and every back after: 1, 3, 5, 7, 9, then the backs. What you need is the 2 files dealt together like cards, 1 page from each in turn.

There is a twist almost everyone hits. Turning the stack over to scan the backs also reverses it: the last sheet now goes through first. So the backs file usually runs 10, 8, 6, 4, 2, not 2, 4, 6, 8, 10.

Are your backs reversed? Check in 10 seconds

Open the backs file and look at its first page.

• The back of your first sheet (what should be page 2): the backs are in order.
• The back of your last sheet, or a blank page: the backs are reversed. This is the usual case when you turned the stack over.

Write down how many sheets you scanned. That number, and whether the backs are reversed, are all you need.

Interleave the 2 files, step by step

Merge, fronts first.

1. Drop the fronts file and the backs file on Merge PDF above.
2. Make sure the fronts file is first in the list; if not, click Down on it.
3. Turn Bookmark per file off. Click Merge and download the result, named after the first file with -merged on the end.

The merged file has every front, then every back. In our test with 5 sheets it ran Front 1 to Front 5, then Back 5 down to Back 1.

Deal them together in Extract pages.

4. Drop the merged file on Extract pages.
5. In Pages, paste the list for your number of sheets from the table below.
6. Click Extract and download. Because the list is not in ascending order, the file is named after it (for 5 sheets, scan-merged-pages-1-to-6-reordered.pdf), so it is never mistaken for the original.

In our 5-sheet test the result came out Front 1, Back 1, Front 2, Back 2, through to Back 5, in 6 milliseconds.

The pattern: each front number (1, 2, 3 and on) is followed by its back. With reversed backs, the back of sheet 1 is the very last page of the merged file and the numbers count down; with backs in order, the back of sheet 1 is the first page after the fronts and the numbers count up.

More than 10 sheets: let a spreadsheet write the list

In Excel 365 or Google Sheets, paste this into any cell. It is the list for 12 sheets (24 pages) with reversed backs:

=TEXTJOIN(", ",TRUE,TOCOL(HSTACK(SEQUENCE(12),SEQUENCE(12,1,24,-1))))

Change 12 to your number of sheets and 24 to twice that. For backs in order, replace SEQUENCE(12,1,24,-1) with SEQUENCE(12,1,13,1), where 13 is your number of sheets plus 1. Copy the cell and paste it into Pages. TOCOL reads the 2 columns row by row, which is what deals the pages together (Google’s and Microsoft’s help pages for TOCOL, checked on 11 October 2026).

Odd page counts and blank backs

A 9-page document printed on both sides uses 5 sheets, and the last sheet’s back is empty. What to type depends on what your scanner did with it.

• It scanned the blank. Both files have 5 pages. Use the 5-sheet list and leave off the last number: 1, 10, 2, 9, 3, 8, 4, 7, 5. In both columns of the table, the last number is always the last sheet’s back.
• It skipped the blank. The backs file has 4 pages and the merged file 9. With reversed backs, type 1, 9, 2, 8, 3, 7, 4, 6, 5; with backs in order, 1, 6, 2, 7, 3, 8, 4, 9, 5.

If several sheets in the middle were single-sided, keep their blank backs while you interleave, then drop the blanks with Remove blank pages. Removing blanks first shifts the pairing: every back after the gap would land next to the wrong front.

Backs upside down?

Depending on which way you turned the stack, the backs can come out upside down. After interleaving, every back is an even page, so 1 pass fixes them all: drop the file on Rotate PDF, choose 180° and Even, and download. The turn is written into each page, not drawn as a new copy, so the scan keeps its quality and the file keeps its size.

The honest part

Interleaving assumes both passes scanned the same stack, complete, in 1 run each. If the feeder pulled 2 sheets at once in either pass, or a sheet jammed and you rescanned it at the end, every page after that point is paired with the wrong partner. No tool can see that from the pages alone; only you know which back belongs to which front.

So check before you send: open the result and look at 2 pages that should face each other, one near the start and one near the end. If the last pair matches, the whole run almost certainly does. If it does not, count the pages in each file: a pass with 1 page too many or too few tells you where the misfeed was. Organise PDF can then move or remove the stray page by hand.

If your files are not 2 scan passes but a stack scanned backwards in 1 pass, you need Reverse PDF page order instead. More page jobs are mapped in Organise PDF pages, and a scan you want to search or copy from needs OCR after interleaving.