PDF Merge — Combine Multiple PDF Files Online
Merge multiple PDF documents into a single file. Drag and drop your PDFs, arrange the order, and download the combined result. 100% client-side — your files never leave your browser.
How do you merge PDF files into one? Merging creates a new PDF containing
every page of every source file, in the order you set. This tool parses each upload with
pdf-lib, copies the pages of the first document into an empty PDF, then
appends the pages of the second, and so on, before saving the result as
merged.pdf. Nothing is re-rendered, so text stays selectable and images keep
their original quality.
How to Merge PDF Files
-
Add your PDFs — Drag files onto the drop zone or use Choose Files to select several at once. Anything reported as
application/pdf, or simply ending in.pdf, is accepted. - Check the file list — Each accepted file appears as a row showing its name and size. The counter in the header confirms how many are queued, and the merge button stays disabled until there is something to merge.
- Set the order — Drag rows up and down. The merged document follows the list from top to bottom, so the first row becomes page 1 — sort before merging rather than fixing the order afterwards.
- Merge the documents — Click Merge PDFs. The progress bar tracks library loading, reading each file, copying pages and writing the output; large scanned files take the longest to read.
-
Download the combined file — Press Download Merged PDF to save
merged.pdf. Use Clear All to empty the queue before starting a different batch.
How Merging PDFs Actually Works
Every PDF is a self-contained object graph. Page dictionaries reference font programs, image streams, colour spaces and graphics states, and a cross-reference table at the end of the file records the byte offset of each numbered object. Two PDFs almost certainly reuse the same object numbers for completely different things, which is why appending one file to another with a file copy produces a broken document rather than a longer one.
A correct merge rebuilds the graph. This tool reads every uploaded file with
FileReader, parses each one into an in-memory document with
pdf-lib, and creates an empty destination document. It then walks the files in
list order and, page by page, calls copyPages, which deep-copies the page along
with everything it references and renumbers those objects so they cannot collide with objects
already in the destination. Each copied page is appended with addPage. When every
page has been copied, save serialises the combined graph and writes a fresh
cross-reference table.
The important consequence is that no content is re-encoded. A JPEG inside page 12 of your third document is carried across as the same JPEG stream, not decoded and re-compressed. Text keeps its font programs and stays selectable and searchable, vector artwork stays vector, and a scanned page is neither sharpened nor degraded. Merging is therefore a lossless operation, and it is not a way to make files smaller — the output is roughly the sum of the inputs.
What Is Preserved and What Is Not
| Element | In the merged file |
|---|---|
| Page content: text, fonts, images, vectors | Copied unchanged, no re-compression |
| Page size, orientation and rotation | Kept per page, so a mixed batch stays mixed |
| Page order | Follows the file list, then page order within each file |
| Annotations attached to a page | Usually carried across with the page |
| Bookmarks and the outline tree | Not carried over — outlines live at document level |
| Interactive form fields | Lose their registration; expect them to stop being fillable |
| Document metadata such as title and author | Not merged; the new document starts clean |
| Cross-document links | Only links whose target page is included still resolve |
Mixed Page Sizes
PDF stores a page size per page, not per document, so merging an A4 report with a US Letter invoice and a landscape spreadsheet produces one file whose pages genuinely differ in size. On screen this is harmless. In print it is not: many printers and print shops apply shrink-to-fit or centre-on-media to the odd pages, which changes margins and can scale a signature block. If the batch is destined for paper, normalise the sources to a single page size before merging.
Encrypted files are refused. If any document in the queue carries encryption — including one with only an owner password limiting editing — the parser rejects it and the merge stops. Open that file with its password, save an unprotected copy, and merge the copy instead.
Frequently Asked Questions
Add every document to the queue, drag the rows so the list reads in the order you want the finished document to read, then click Merge PDFs. The tool builds one new file containing all the pages and offers it as a download. The source files on your disk are never modified — merging only ever produces a new document.
There is no fixed cap. Every file is held in memory at once, so the real limit is the RAM available to the browser tab. Dozens of ordinary text documents merge comfortably; a handful of large scanned files can be heavier than a hundred small ones. If the tab becomes unresponsive, merge in two passes and then merge the two results.
Neither. Pages are copied at the object level with no decoding or re-compression, so images keep their original resolution and text keeps its embedded fonts. That also means merging cannot shrink anything: the output is approximately the combined size of the inputs. To reduce size you need a compression step, which is a different operation entirely.
Page content is, but document-level structure is not. Bookmarks and the outline tree belong to the document rather than to any page, so they do not survive the copy. Interactive form fields lose the catalogue entry that makes them fillable, and links pointing at pages from a file you did not include have nothing left to resolve to.
The documents are read by your browser and merged in the page you are looking at; their contents never travel over the network. The pdf-lib script is fetched from a public CDN the first time you merge, which carries none of your data. Nothing is stored, so closing the tab leaves no trace of the documents.
Yes to both. Scanned pages are image streams and are copied like any other page content. Different page sizes are preserved individually, so the merged file can legitimately contain A4, Letter and landscape pages side by side. That is fine on screen but can cause scaling surprises at the printer, so normalise sizes first if the result is going to paper.
Merge when several files should become one. PDF Split when one file should become several. PDF Extract Pages when you want a specific handful of pages pulled into a new document. A common workflow uses two of them: extract the pages you want from each source, then merge the results.
The output follows the file list top to bottom, and within each file its own page order — not the order you happened to select the files in the browse dialog, and not alphabetical order. Reorder the rows by dragging them before pressing Merge. If a single source file has pages out of order, fix that file first with a page reorder tool.
Use Cases
Contract Assembly
A paralegal combines the signed signature pages returned by four parties with the base agreement and its exhibits, producing one execution copy for the file.
Reading List Compilation
A postgraduate merges eight downloaded papers into one file for a tablet, so a week of reading opens as a single document instead of eight separate tabs.
Month-End Invoice Packs
A bookkeeper merges the twelve supplier invoices received as separate attachments into one PDF before uploading a single document to the accounting system.
Thesis Submission
A research student assembles separately written chapters, the appendices and the bibliography into the single file a submission portal demands.
Job Application Bundles
An applicant merges a CV, a cover letter and scanned certificates into one attachment, because the recruiter's form accepts only a single upload.