How to combine multiple PDFs into one file

Merging PDFs is one of those small but oddly frequent tasks. You need to combine a signed contract with its attachments, assemble a reference document from several reports, or stitch together scanned pages that got split into separate files. The task sounds trivial — “stick them together” — and it is, but there are a few things worth knowing to get it right every time.

The basic idea

A PDF file is a container of objects: pages, fonts, embedded images, metadata. Merging two PDFs means taking the pages from file A, the pages from file B, and creating a new PDF that contains both sets of pages in sequence.

This is not the same as concatenating the raw file bytes (that produces an invalid file), printing both to a single PDF (which re-rasterises text into images, bloating size and killing searchability), or taking screenshots (same problem, worse). A proper merge keeps the text as text, fonts as fonts, and embedded images as embedded images. The result is clean, searchable, and roughly the same size as the sum of the inputs.

Step-by-step: the standard workflow

For most situations:

1. Collect the files you want to merge into a folder
2. Decide the order — which file goes first, which last
3. Check for password protection on any of the files — you’ll need to remove the password before merging
4. Open a PDF merger (see below)
5. Add the files in your chosen order
6. Download the merged result
7. Verify the output contains all expected pages in the right order

For step 4, you have roughly three categories of tool. Desktop software (Acrobat, PDFsam, open-source equivalents) is powerful but requires install and often a license. Online services are fast but your documents get uploaded to a server — think about whether that’s acceptable for the content. Browser-based tools that run locally need no install, no upload, no server. That’s what the PDF Merge tool does, and it’s what most people should default to.

Using a browser-based merger

Here’s the full process with Filesty’s PDF Merge:

1. Open the page. It loads in under a second — it’s static HTML.
2. Drop your PDFs onto the page or click Browse to pick them. Add as many as you need.
3. Reorder by removing files from the list and adding them back in the order you want.
4. Click Merge PDFs.
5. Click Download.

Everything happens in your browser using a WebAssembly PDF library. The files never leave your device. No upload, no temporary storage on someone else’s server, no account required.

Getting the page order right

The page order of the merged PDF matches the order of files in your list. If you add contract.pdf (2 pages), then appendix-a.pdf (5 pages), then signatures.pdf (1 page), the result has 8 pages in that exact order.

A few things to watch out for. Case sensitivity: filenames are sorted case-sensitively in some pickers. Appendix.pdf and appendix.pdf may sort differently than you expect. Numbered prefixes: if you name files 1-intro.pdf, 2-body.pdf, 10-conclusion.pdf, they’ll sort alphabetically as 1, 10, 2 — not 1, 2, 10. Prefix with zeros: 01, 02, 10. Mixed page orientations: if one source is portrait and another landscape, the merge respects the original orientation of each page.

Password-protected PDFs

If any input PDF has open-password protection (you need a password to view it), merging fails — the tool can’t read the pages without decrypting them. You need to either remove the password first using a PDF editor (you’ll need the password to do this), or ask for an unlocked copy from whoever sent it.

Owner-password protection (which restricts editing/copying but allows viewing) is usually fine for merging, depending on the specific restrictions set.

What happens to bookmarks, annotations, and form fields?

Different tools handle these differently. A browser-based merger focused on pages and basic content will typically preserve pages, text, images, fonts, and per-page dimensions — but lose bookmarks, annotations, form fields, and digital signatures. Encryption is also not preserved (output is unencrypted).

If you need to preserve bookmarks, annotations, or digital signatures, use a full-featured desktop PDF editor instead.

File size considerations

A naïve merger that concatenates pages without de-duplicating shared resources can produce a file larger than the sum of the inputs — for example if the same font is embedded in each source file, it ends up embedded multiple times in the output. Good PDF libraries de-duplicate shared resources where possible. The typical result is that the merged file is about the sum of the inputs, maybe 10% smaller if there’s significant font overlap.

If your merged PDF comes out much larger than expected — say, double the input sum — something in the tool is re-rasterising pages. Use a different tool.

Verifying the output

After a merge, check:

1. Total page count — should equal sum of input page counts
2. First and last pages of each section — are they the right content?
3. Text is still selectable — if you can’t select text that was selectable in the source, the output got rasterised
4. File opens in at least two different PDF viewers — Adobe Acrobat, your browser’s built-in viewer, macOS Preview. If it opens everywhere, it’s a valid PDF.

Common mistakes

Uploading confidential PDFs to random online tools. This is by far the biggest risk. A surprising number of “free” online PDF tools keep and analyse the documents they receive. Business contracts, legal documents, medical records, tax returns — don’t upload these anywhere unless you know exactly who’s running the service. If in doubt, use a tool that runs locally in your browser.

“Printing” to PDF to merge. Some people use “Print → Save as PDF” and print multiple documents into one. This destroys the text layer (everything becomes an image), inflates file size 5-10×, and breaks searchability. It’s a last resort, not a workflow.

Merging then forgetting to verify. A failed merge sometimes produces a file that looks right but is missing pages or has them out of order. Always check the total page count.

Forgetting the password. If you need to send a merged PDF that should be password-protected, add the protection after the merge. Don’t expect the merger to preserve encryption from the source.

---

The core idea is simple: merging combines pages from multiple PDFs into a single file, in order. Order matches the order of files in your list, so rename with leading zeros to sort properly. Browser-based tools are safest for confidential content — nothing uploads. Bookmarks, annotations, and signatures usually don’t survive a simple merge. And always verify the page count after.

If you have a set of PDFs to combine right now, drop them into the merger. It works entirely in your browser, handles files of any reasonable size, and doesn’t send anything to any server. For anything confidential — never upload. Merge locally.