How it works
- You add two or more PDFs. Each one is read with FileReader into an ArrayBuffer held by this tab. No network request is made at any point, and nothing is written to disk or to storage.
- pdf-lib parses each buffer into a document object and hands back the page tree. Merging is not a re-render: the pages of file two onward are copied as page objects into file one's document, with their content streams, embedded fonts and image XObjects carried across intact.
- The order you set in the staging list is the order pages are copied in, so the output page numbering follows the list from top to bottom. Moving a file up or down reorders the copy queue, not the pages inside that file.
- Copying runs inside a Web Worker so a 400-page merge never blocks the main thread. Progress in the panel is driven by real messages from that worker as each source document finishes, not by a timer.
- The finished document is serialised once and wrapped in a blob URL held only by this tab. You download it, and closing or reloading the page revokes the URL and discards both the inputs and the output.
Limitations
Form fields and signatures are flattened. A merged PDF keeps the visual appearance of a filled form, but the AcroForm dictionary that made the fields fillable does not survive being copied into a different document, and a digital signature is invalidated the moment the byte range it signed changes. If you need the form to stay interactive or the signature to stay verifiable, merge before signing, not after.
Bookmarks and internal links from documents two onward are dropped. The outline tree and any link annotations that point at a page by object reference no longer resolve once that page lives in a new document. Page labels — the roman-numeral front matter some reports use — are also reset to plain arithmetic numbering. Tagged-PDF structure for screen readers survives inconsistently and is worth checking if accessibility matters for the output.
There is no page-size normalisation. If you merge an A4 report with a US Letter appendix, the output contains both sizes, exactly as the sources had them. Printers usually handle this by scaling, but a print shop may charge for the inconsistency, so it is worth knowing before you send the file.
Questions
Is there a file size limit?
No cap is enforced, and you can add several files at once. The practical limit is your device's memory — the whole set has to fit in the tab at the same time, so a few hundred megabytes is safe on a phone and a couple of gigabytes is realistic on a desktop. If a merge fails silently, that is almost always what happened.
Why did my filled form stop being fillable?
Because merging flattens it. The fields render as they looked when you added the file, but they are no longer editable. Merge the blank templates first, then fill the combined document, or keep the fillable original alongside the merged copy.
Does the file size add up exactly?
Usually it comes out slightly under the sum. Fonts embedded in more than one source are deduplicated when the pages are copied, so a set of documents produced from the same template can lose a few percent. It is not compression — if you need the file smaller, run the result through Compress PDF.
Can I merge a password-protected PDF?
Not while it is encrypted. Open it in your PDF reader, remove the password, save a copy, and add that copy here. We do not ask for the password because there is nowhere on this page for it to go safely.
Do I need an account?
No. There is no sign-up, no email step and no usage counter. The tool is paid for by the ads on this page.
When people use it
Most people land on Merge PDF with one job and one deadline: a lender wants the application, the salary slips and the bank statement as a single attachment, or a court filing has a page limit on the number of documents rather than the number of pages. The staging list is the point of the tool — the order is the thing that has to be right, and getting it wrong means the reviewer reads an annexure before the contract it belongs to.
The other common shape is scanning. Phone scanner apps produce one PDF per session, so a ten-page lease that took three sittings arrives as three files. Merging them back into one document, in order, is a thirty-second job that otherwise means opening desktop software you may not have on the machine you are sitting at.
Some of what needs combining is often not a PDF yet — a page photographed on an iPhone arrives as HEIC, which nothing here can place directly. HEIC to JPG converts those first, and the results can then be assembled into a PDF and merged with the rest.
How it compares
A server-side merge tool cannot promise you anything about a contract you upload beyond a retention policy you have to take on trust; this one never sends the file, so there is nothing to retain. What a desktop application does better is everything after the merge — Acrobat and its open-source equivalents can rebuild the outline tree, renumber page labels and re-apply a signature, and if the merged document is going to be a permanent record rather than a one-off attachment, that work is worth doing there.