Merge PDF

How to use Merge PDF

Key features of Merge PDF

About Merge PDF

Merge PDF takes two or more PDF files and copies every page from each into one new document, in the order you set. It uses the pdf-lib library in your browser, so page content, page sizes and rotation are carried over unchanged and nothing is re-rendered. Bookmarks are not copied into the merged file.

It is a quick way to join a scanned contract with its signed last page, put a cover letter in front of a CV, or turn a folder of monthly statements into one file. Password-protected PDFs are marked with a lock in the list and cannot be merged, so remove the password first.

When to use Merge PDF

A lease came back as three PDFs: the agreement itself, the signed signature page, and an addendum scanned later. Add all three, use the arrows to put the signature page after the agreement, and merge. The tenant receives one attachment instead of three, and the pages inside each file keep their original order.

A year of bank statements downloaded as twelve files, one per month. Select all twelve in the file picker at once; they appear in the list in the order the picker handed them over, so check that January is at the top and fix any that are out of place before you click Merge PDFs. The first-page thumbnails help with that.

Two PDFs where only part of the second one belongs in the result, such as a proposal plus the pricing pages of a longer catalogue, are a job for Combine PDF. It takes a page selection for each file, so you skip the step of merging everything and deleting the excess afterwards. Merge PDF is the quicker choice when every page of every file belongs in the output.

If you already have one file and want to change what is inside it, this is the wrong tool. Delete Pages removes pages, Reorder Pages changes their sequence, and Extract PDF Pages copies a few of them into a new file.

Tips and troubleshooting

• The Merge PDFs button is greyed out until the list holds at least two files. With one file there is nothing to join; add another with + Add More Files.
• The line under Files to Merge adds up the pages. In a long list, rows get their thumbnails as you scroll to them.
• A file pdf.js cannot open, such as a damaged file or one that is not a PDF (drag and drop accepts any file type), shows a warning sign and "Could not read this file." If pdf-lib cannot read a file when you merge, nothing is merged and a red note names it; remove it and click Merge PDFs again.
• A PDF that needs a password shows a lock and "Password protected. Unlock it first." One that opens but carries owner restrictions, such as no printing, keeps its thumbnail and gets a note. pdf-lib refuses both: remove the protection with Unlock PDF and add the unlocked copy.
• A file over 150 MB is refused before it reaches the list, with a red note naming it (above the list when you use + Add More Files). None of the files chosen in that batch are added; files already in the list stay.
• The output is always named merged_document.pdf, whatever the inputs were called. Its size is close to the inputs added together, because each file's fonts and images are copied across; Compress PDF can shrink the result.
• Adding the same file twice puts its pages in twice. That is allowed, and it is one way to get two copies of a form in one document, but check the list before merging if it was not intended.
• Process Another File reloads the page, which empties the list. Download the result first.
• The thumbnails need the pdf.js worker, which the page fetches from cdnjs.cloudflare.com once it has loaded. If that fetch fails, the list shows plain page icons and no page total, and merging still works.
• The guide how to merge PDF files without uploading them covers locked files, partial merges and what to do when the result is too large.

How Merge PDF works in your browser

When you add files, the list shows their names and sizes. pdf.js, running in a Web Worker, then opens them one at a time to count the pages. For rows on or near the screen it draws the first page with the print intent into a box 72 by 80 pixels (up to twice that on high-density screens), keeps it as a small PNG and closes the document to free memory. A drawn thumbnail is kept for its file, so moving or removing rows never draws it again.

On Merge PDFs, the thumbnails pause and the script creates an empty document with pdf-lib, then goes through the list from top to bottom: it reads each file into memory again, opens it with pdf-lib, copies every page (with the fonts, images and other objects the page refers to) into the new document, and moves on to the next file. The progress bar advances one step per file and shows the name of the file being copied.

The merged document is then serialised to a byte array and wrapped in a Blob, which is what the Download File button hands to your browser under the name merged_document.pdf, through a temporary object URL. Preview Document draws the pages of the same Blob with pdf.js in a modal window, or shows them in your browser's own PDF viewer when pdf.js cannot. No bytes leave the tab: there is no upload, no server-side step and no copy stored anywhere, and the memory is released when you close or reload the page.

Frequently asked questions