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5 min read · Updated 2026-09-30

How to merge PDF files

Joining documents copies pages into a new PDF rather than re-rendering them. This guide covers the actual order controls and the document-level features that a merge cannot promise to preserve.

Merging two or more PDFs is concatenation: each page in your sources is an object that gets copied wholesale into a new document. Nothing is re-rendered, so text stays selectable and images keep their original data. What needs care is not the copying but the structures that span a whole document — outlines, form fields and signatures.

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Tools used in this guide

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5 steps

The procedure

Steps for a MyPDFilles tool run in your browser tab. When a guide directs you to external software, its own privacy and security practices apply.

  1. 01

    Add two or more PDFs

    Drop the files onto the drop zone or use Choose files. Merge PDF needs a minimum of two, and you can keep adding files after the first batch without starting over.

  2. 02

    Put the stack in the right order

    Use the arrow buttons beside each file to move it earlier or later. The list read top to bottom is exactly the page order of the result — there is no separate sort step.

  3. 03

    Decide about bookmarks

    Add a bookmark for each file is on by default. It creates one flat bookmark per source file at that file’s first page. Existing source outlines are not copied or nested into the result.

  4. 04

    Decide about right-hand pages

    If the result will be printed double-sided, enable the start-on-odd option. A blank page is inserted wherever needed so each document begins on an odd page and no section starts on the back of another.

  5. 05

    Merge and download

    Press Merge PDF. The work runs on your own device, so the wait depends on your machine rather than a server queue, then save the combined file.

Beyond the steps

What is actually happening

01

Form fields are not guaranteed to survive as usable forms

An interactive PDF form stores its values in document-wide field structures rather than entirely in page content. When pages are copied into a new document, those structures may not remain available or may not behave as they did in the source.

Do not rely on this merge to preserve a fillable form. If the visible completed values are what you need to share, flatten the form first in software you trust: that paints the current appearance into the page, but it does not sign the page or make it tamper-proof.

If you need an editable combined form, use dedicated form-authoring software and test the resulting fields before distribution.

02

Digital signatures are not preserved

A cryptographic signature covers bytes in a particular file. A merge makes a new file with a new page tree and cross-reference structure, so signatures from the inputs are not preserved as valid signatures in the result.

If a signature must remain verifiable, keep the signed file intact and distribute it alongside any combined reading copy. A visible signature appearance is not proof that the original digital signature still validates.

03

Mixed page sizes stay mixed

Every PDF page carries its own MediaBox, so page size is a per-page property rather than a document setting. A4 is 595 × 842 points and US Letter is 612 × 792 points — Letter is wider and shorter — and 72 points is one inch. Merge an A4 report with a Letter appendix and the result concatenates those page sizes in the source order; it does not alternate them unless your files already alternate them.

That behaviour is deliberate: normalising sizes means scaling page content, which changes how the pages look. If you need one uniform size, run the sources through the page-size tool first and merge afterwards, so the scaling decision is one you made explicitly.

04

Do not expect cross-file de-duplication or a size saving

The output is a new PDF, so its byte size can differ from the total of its inputs. This merge copies resources from separate source files as separate references; it does not automatically de-duplicate matching fonts, images or other resources across files.

Do not treat a merge as a size-reduction step or promise a particular output size. If you need a smaller reading copy, merge first, then use a compression workflow and inspect the resulting quality.

While you follow this

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Questions about this task

How many files and pages can I merge at once?

The current intake limits are 250 files, 250 MB per input file, 600 MB total across the selected files, and 5,000 pages per input file. Available browser memory can impose a practical limit before any of those ceilings.

Does merging reduce the quality of my pages?

Pages are copied into a new document rather than re-rendered. Check the result before sharing, especially if the inputs contain forms, annotations or other document-level features.

Can I merge a password-protected PDF?

No. This merge rejects encrypted inputs and does not ask for a password. Decrypt a file you are authorised to open in appropriate software first, then merge the decrypted copy if that is suitable for your workflow.

Will matching fonts or images be de-duplicated?

Not automatically across separate input files. The merged output may be larger or smaller than you expect, so do not use it as a size promise.

Is merging the same as combining?

Mechanically yes — both concatenate page objects. The Combine PDF page is oriented toward assembling a deliberate multi-section package, so it leads with section order and double-sided printing.

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