How to merge PDFs into one file (and get the page order right)

A single logical document has arrived as several separate files, and whoever is receiving it wants one attachment. This is one of the most common PDF tasks there is, and it comes up in a few reliable shapes.

• An expense claim built from a dozen individually scanned receipts
• A contract where the signed signature page came back on its own
• A report where four people each wrote a section and exported separately
• A mortgage or visa application needing payslips, statements and ID in one submission

Merging beats copying and pasting between documents, because nothing is re-rendered. Fonts, tables, embedded images and page sizes all survive exactly as the original authors set them.

How to merge

1. Add all the PDFs you want to combine — you can usually select several at once.
2. Check the order in the file list, because that is the order the pages will appear in.
3. Drag files up or down to reposition them.
4. Remove anything you added by mistake.
5. Merge and download the combined document.

Getting the order right without the dragging

When you select many files at once, most browsers hand them over in filename order. That is a lever worth using: renaming files with numeric prefixes before you select them means the list arrives already sorted.

Prefix your filenames 01-, 02-, 03- before selecting them, and the order sorts itself. Note the leading zero — without it, 10 sorts before 2.

That leading zero detail catches people out constantly. Alphabetical sorting puts "10" before "2" because it compares character by character. With 01, 02, 10 the ordering is correct.

For a handful of files, dragging is faster than renaming. For twenty scanned receipts, renaming is much faster than dragging.

Three things that commonly go wrong

Password-protected files will not merge. An encrypted PDF cannot be read until it is decrypted, so the merge either fails or silently skips it. Unlock the file first, then merge the decrypted copy.

Mixed page sizes stay mixed. Merging never resizes anything, so a combined file containing A4 pages and US Letter pages keeps both. On screen this is barely noticeable; printed, it looks untidy. If consistency matters, standardise the source documents before merging.

The result can be very large. Merging ten image-heavy scans produces one substantial PDF, which may bounce off an email limit. Merge first, then compress the finished document once — compressing the parts individually and then merging is more work for a worse result.

Merging things that are not PDFs

Merge tools take PDFs only. If some of your material is in another format, convert it first:

• Photos and scans from a phone: convert with JPG to PDF, which also lets you set a consistent page size.
• Word documents: convert with Word to PDF, which preserves your layout.
• Spreadsheets and slides: convert those too, checking the page setup before you do.

Doing the conversions first and merging once at the end is considerably less painful than discovering halfway through that one file will not go in.

After merging

Two finishing touches are worth considering for anything that will be read by someone else.

Add page numbers. A document assembled from five sources often contains five separate numbering sequences, or none at all. Stamping a fresh continuous sequence over the finished document makes it referenceable, and it should always be done after merging rather than before.

Check the joins. Skim the pages either side of each boundary between source documents. Missing pages and duplicated cover sheets are easiest to spot at the seams, and much easier to fix now than after you have sent it.