Maker note: I built All Done — a free set of browser-based tools, including the Merge PDF tool mentioned below. This post is my rewritten version of our guide on all-done.xyz.

Merging PDFs sounds like the simplest job in the world — until the file you send back has the pages in the wrong order, or your email provider refuses it for being too big. The good news: the actual merge takes about half a minute. The rest is just avoiding a few easy mistakes.

The 30-second method

  1. Open a merge tool in your browser — no install, no account.
  2. Drop your PDFs in.
  3. Drag them into the order you want.
  4. Hit merge, then download the single combined file.

That is the whole process. Everything below is about not having to do it twice.

Mistake 1: Pages end up in the wrong order

Uploads do not always land in the order you expect, and a report that reads 1, 3, 2 looks sloppy at best and confusing at worst. Before you merge, look at the preview and drag the files into sequence. A small habit that helps: rename your files with numbers first — 01-intro.pdf, 02-body.pdf, 03-appendix.pdf — so the order is obvious before you even upload.

Mistake 2: The merged file is too big to send

Three 25 MB scanned PDFs become one 75 MB file, and Gmail will bounce anything over 25 MB; WhatsApp will struggle too. Check the total size before you merge. If it is bloated, shrink the individual files first: a 300-DPI color scan runs about 1 MB per page, so a 50-page scan is 50 MB on its own, and running it through a PDF compressor first can cut it by 70–80% with no visible difference on screen.

Mistake 3: Trusting the wrong tool with sensitive documents

Merges often involve contracts, bank statements, medical records, or visa paperwork — and every file you upload is a file handed to someone else's server. That does not mean avoiding online tools; it means picking honest ones. Good signs: the tool processes files in your browser so nothing is uploaded at all, or it states a clear auto-delete policy with files erased within the hour. Warning signs: mandatory accounts, watermarks on free merges, and privacy pages that will not say what happens to your files. For truly sensitive documents, an offline desktop tool remains the safest option — and as a rule of thumb, if the merge involves personal data like IDs, signatures, or account numbers, prefer a tool that never uploads at all.

Mistake 4: Merging your only copies

Merging is non-destructive in a good tool, but things still go sideways — a wrong file gets included, or a download fails halfway. Keep your originals. A backup habit costs nothing and saves everything when something goes wrong.

Mistake 5: Sending the result without checking it

The merge finished, so it must be fine — right? Not always. Open the merged file once before you send it: confirm the page order reads 1, 2, 3, check the total size is under your email limit, and make sure nothing is missing. Thirty seconds of checking saves the re-do.

Before you hit merge — the checklist

  • [ ] Order checked in the preview — reads 1, 2, 3, not 1, 3, 2.
  • [ ] Total size under your email limit (compress first if it isn't).
  • [ ] Originals kept — never merge your only copies.
  • [ ] Tool chosen: no signup, no watermark, clear privacy policy.

Try it free

Merge PDF on All Done — free, no signup, no watermark, and your files are processed in your browser, never uploaded to a server.