How to merge PDF files

The short answer

Open the files, put the pages in the order you want, save one PDF. It takes about fifteen seconds and nothing should happen to the pages along the way.

The part worth understanding is that last clause, because it is where most tools quietly fail.

Merging should copy pages, not redraw them

A PDF page is not a picture. It is a set of drawing instructions, text as text, with the font embedded, vectors as vectors, plus whatever images are placed on it. Merging done properly copies those instruction streams from one document into another and changes nothing about them.

Merging done badly renders each page to a bitmap and writes the bitmaps into a new file. The result looks almost right and is much worse:

• The text stops being text. You cannot search it, select it, or copy from it,

and a screen reader finds nothing to read.

• Vectors become pixels, so the page is sharp at one zoom level and soft at

every other.

• The file gets bigger, often several times over.
• Form fields, links and bookmarks disappear.

You can check in seconds: open the merged file and try to select a line of text. If the cursor will not catch on a word, the page was flattened.

Ours copies. Each page is lifted out of its source document and placed into the new one intact, which is why merging a searchable PDF gives back a searchable PDF.

Order is the whole job

Almost every real merge is "these files, but page 7 of the second one goes first", and that is where tools that only reorder whole files stop being useful. Reordering at the page level matters because the thing you are assembling usually does not arrive in neat documents.

If the tool you are using can only stack files end to end, you will end up splitting first and merging after, which is two operations and a folder of intermediate files to clean up.

What "free online" usually costs

Most free PDF sites work by uploading your file to a server, doing the work there, and handing back a link. That is a reasonable engineering choice and a bad idea for a contract, a payslip, a passport scan or a medical letter, which is most of what people merge.

Things worth knowing before you upload:

• The file sits on a third party's disk, for a retention period you are

trusting rather than verifying.

• Many such services cap the free tier by file size or page count, and the cap

is usually discovered halfway through.

• Some add a footer to every page.

A merge does not need a server. The arithmetic is small and every browser can already do it, which is why this one runs on your machine and the file never leaves it.

Things that legitimately change

Honesty about the edges, because "nothing is touched" is not quite true:

• The document-level metadata is new. Title, author and creation date

belong to the document, not the pages, and there are now several sources for them. The merged file gets its own.

• Bookmarks and outlines are document-level too. Page content survives;

a table of contents that pointed into the old file may not.

• Form fields with the same name across two files collide. Two documents

each containing a field called signature cannot both keep it.

• Encryption does not survive. A password-protected source has to be

unlocked before its pages can be copied anywhere.

If the file will not open

Two common causes, both fixable:

It is password protected. A PDF can be encrypted for opening or only for editing. Either way the pages cannot be read until it is unlocked, and no merge tool can bypass that, nor should one.

It is not really a PDF. A file renamed from .docx to .pdf is still a Word document. So is a .pdf that a scanner produced and truncated halfway. If every tool refuses it, the file is the problem rather than the tool.