Merge PDF Files

A PDF is a container: a list of page objects plus the fonts, images and metadata those pages refer to. Merging is not conversion — it is building a new container and copying the existing page objects into it, in order, along with everything each page depends on.

That is exactly what happens here. Each file you choose is read into memory with the browser's FileReader, parsed by pdf-lib, and its pages are copied into a fresh document one after another. The result is saved to a blob and offered as a download. There is no upload step because there is no server in the design — the entire operation is arithmetic on bytes that never leave the tab.

The practical consequence is that page content is copied, not re-encoded. Text stays selectable, vector graphics stay sharp at any zoom, and an image that was 300 dpi in the source is still 300 dpi in the output. A tool that rasterises pages to merge them would quietly cost you all three.

Files are merged in the order they appear in the list, and every page of each file is included. Nothing is reordered, dropped or resized, and pages of different sizes stay at their own sizes — merging an A4 contract with a US-Letter receipt gives you one document containing both, each page keeping its original dimensions.

Because pages are copied rather than rebuilt, the output size is roughly the sum of the inputs. Merging is not compression — if the result is larger than you want, compress it afterwards rather than expecting the merge to shrink it.

The tool needs at least two files, which is the only rule it enforces. Beyond that the limit is your device's memory, not an artificial page or file-size cap: a few hundred pages of text merges in a second or two, while a folder of high-resolution scans will take longer and use more RAM on an older phone.

Because it is easier to build that way, and because it was the only way to build it for most of the web's history.

Manipulating PDFs used to require server-side libraries, so the standard architecture was: take the file, POST it, process it on a machine you control, return a download link. That link usually lives at a public URL for some period. The file sits on a disk. It appears in logs. It may be replicated to a backup. None of that is necessarily sinister — it is simply what the architecture implies — but it means your document has been handed to an organisation you have not audited, under a retention policy you have not read.

Now think about what people actually merge. Signed contracts. Scans of a passport for a visa application. Medical letters. Bank statements for a mortgage broker. Invoices with client names on them. This is close to a worst case for casual uploading, and it is precisely the category where the convenience of a free web tool is most tempting.

This page cannot leak your document because it never receives it. That is not a promise about our retention policy — it is a property of where the code runs, and you can verify it in about ten seconds. Open your developer tools, switch to the Network tab, merge two files and watch: no request carries them. Or load the page, disconnect from Wi-Fi, and merge anyway. It still works.

Merging copies pages, and a few things live outside the pages. Bookmarks — the clickable outline some PDFs show in a sidebar — belong to the document as a whole rather than to any page, and they are not reconstructed in the merged file. Document-level metadata such as the original title and author is likewise not inherited; the output is a new document.

Password-protected files are rejected rather than silently mangled. If a PDF is encrypted you will get an error, and the fix is to open it in a viewer, supply the password, and save an unlocked copy first. A tool that quietly stripped encryption for you would be doing something you did not ask for.

Interactive form fields are copied with their pages, but two source files that use the same field names can collide once they share one document, which is a limitation of the format rather than of this tool. If you are merging filled forms and the values matter, flatten them before merging.