Combine Double-sided Scans

How to combine double-sided scans into one PDF

A scanner that scans one side at a time gives you two piles: all the fronts (pages 1, 3, 5…) and, after you turn the stack over, all the backs (…6, 4, 2), usually the last one first. This puts them back as 1, 2, 3, 4…

ముందు పేజీలు ఒక ఫైల్‌లో, వెనుక పేజీలు మరో ఫైల్‌లో ఉన్నా సరైన క్రమంలో ఒకే PDFగా కలుపుతుంది.

1. Say how you scanned: two files (fronts, then backs) or one file with all the fronts first. Photos of the pages work too: choose them all for each side.
2. Choose the files. A locked PDF opens with its password.
3. The order is found for you (below). Check the pages on the right read 1, 2, 3… Drag a page to move it (on a phone, hold it a moment first), tick a page to leave it out, ⇅ to turn it upside down.
4. Tap Save, then download or share the PDF.

Everything happens in your browser: your scans are never uploaded to ap7am or anywhere else. The new file is named after the fronts, such as certificates-combined.pdf.

How the order is found

• Page numbers. Where the PDF has text (a scan made searchable, or a printed PDF), the page number printed at the top or foot of each page is read: "7", "- 7 -", "Page 7 of 20", "7/20", "పేజీ 7", Telugu digits. Both ways of putting the backs in are tried, and the one whose numbers run on is used, when it clearly wins; the page says so.
• No text? A plain scan is only pictures, so there are no numbers to read (reading them would mean recognising the text, slow on a phone). You see two small pictures of the first sheet, page 1 with the back each way would give it and then page 3: pick the one that reads 1, 2, 3. Most scanners give the backs last page first, so that is the start.
• Upside-down backs (the stack turned over top to bottom instead of side to side) are turned for you when their text runs upside down; otherwise tick Turn the backs the right way up.
• An odd number of pages: when there is one back fewer than fronts (the last sheet's blank back was not scanned, or the scanner skipped it), the last front ends the PDF on its own. With one file, set where the backs start if the page has not found it.
• Blank backs are found as Remove Blank Pages finds them (faint show-through and specks do not count as ink) and marked "Left out"; tap one to keep it.

What it keeps, and what it cannot do

Every page is copied as it is: its text stays sharp and searchable and its pictures keep their quality; only the order (and any turn you ask for) changes. A locked PDF is opened with your password and copied the same way; the new PDF has no password (Protect PDF can lock it again). A PDF whose author set limits on changes goes in as pictures unless you take the limits off first with Unlock PDF and its owner password. As when pages are rearranged anywhere, bookmarks and a fillable form are not carried across.

It cannot tell the order of a scan without text from the pictures alone: a sheet's front and back share nothing a computer can match reliably. That is why it shows you the first sheet both ways.

Questions

Made with pdf-lib (MIT licence), Mozilla's pdf.js (Apache 2.0 licence) and qpdf (Apache 2.0 licence, for locked PDFs), all running in your browser. Licences.