Merge PDFs

About PDF Merger

This tool merges multiple PDF files into a single PDF document. You can upload multiple PDFs, reorder them as needed, and combine them into one file. All processing happens in your browser for complete privacy. See also our Compress PDF and PDF to Image.

How do I combine multiple PDFs into one file?

Drag and drop the PDFs you want to merge onto the upload area, or click to select them from your file picker. Once they appear in the list, drag the tiles to reorder them — the final document follows that order top to bottom. Click the Merge button and the tool stitches every page of every file into a single PDF, preserving each source's page size, embedded fonts, hyperlinks, and bookmarks. The merged file downloads directly to your browser; nothing is sent to a server, so even confidential contracts stay on your machine. You can repeat the process with the result file to append more pages later, or split it back apart with the Split PDF tool whenever you want to undo the merge.

Is there a file-size or page-count limit when merging PDFs?

Because merging happens entirely in your browser using pdf-lib, the practical limit is your device's available RAM rather than a fixed server quota. On a modern laptop with 8 GB of RAM, you can comfortably merge dozens of files totalling several hundred megabytes; on a phone, expect to handle around 100 MB before the tab slows. Each input PDF is decoded, its object tree copied, and the cross-reference table rewritten — so a single 500-page scanned PDF uses more memory than ten 50-page text PDFs of the same total bytes. If you hit a limit, merge in batches: combine groups of five, then combine the results. The output is always a valid PDF 1.7 file that opens in every reader.

Will merging preserve bookmarks, hyperlinks, and form fields?

Internal page links and most external hyperlinks are preserved because pdf-lib copies the annotation dictionaries together with the pages they belong to. Bookmarks (the PDF outline tree) are flattened — the merged file keeps the table of contents entries that point to surviving pages, but cross-document bookmarks pointing to removed pages are dropped. Interactive form fields (AcroForm) are kept, but if two source PDFs use the same field name, only one wins, which can scramble field values. For form-heavy merges, flatten the forms first using the Sign PDF or a print-to-PDF step. Digital signatures are invalidated by any merge by design — that is the whole point of a signature — so re-sign after combining if you need legal validity.

Can I merge password-protected PDFs without unlocking them first?

No — pdf-lib needs to read every object inside each input file, and encrypted PDFs hide their content stream and cross-reference table behind a user or owner password. Run the Unlock PDF tool first if you legitimately own the file or know its password, then feed the cleartext result into Merge PDFs. The merged output itself is unencrypted by default; if you want to protect the combined file, run it through Password Protect PDF afterwards and pick the encryption level you need (40-bit RC4 for legacy readers, 128-bit AES for general use, or 256-bit AES for sensitive material). Keeping unlock and merge as separate steps gives you an audit trail and lets you re-encrypt with a fresh key.

How do I merge only specific pages from each PDF rather than the whole file?

The Merge tool combines whole documents, so the cleanest workflow is to use Extract PDF Pages first on each source to produce a smaller PDF containing only the pages you want, then feed those trimmed PDFs into Merge. For example, take pages 3–7 from contract A and pages 1, 5, and 9 from report B, then merge the two extracts into a single briefing pack. You can also use Delete PDF Pages on a copy of each source to discard what you do not need. Both approaches are lossless — pages are copied byte-for-byte, no re-encoding — so the merged file is visually identical to the originals and keeps the same OCR text layer if there was one.

How does merge handle different page sizes like A4, Letter, and Legal mixed together?

Each page in a PDF carries its own MediaBox, so the merged file faithfully preserves the original dimensions: page 1 might be A4 (210×297 mm), page 2 Letter (216×279 mm), and page 3 Legal (216×356 mm). Most viewers render them at actual size, scrolling smoothly between formats. If you need a uniform output for print, run the merged file through Resize PDF to rescale every page to a single MediaBox, or use a virtual printer like Microsoft Print to PDF set to a fixed paper size — both approaches re-rasterise vector content, so prefer Resize PDF when text crispness matters. ISO 32000 explicitly allows mixed page geometries, which is why archival PDF/A-2 and PDF 2.0 both accept the same heterogeneous structure.

Does merging produce a linearized (Fast Web View) PDF for streaming over HTTP?

The default merge output is not linearized — pages are written in the order pdf-lib copies them, with the cross-reference table at the end of the file, which is the simplest valid PDF 1.7 layout. To produce a linearized file (sometimes called Fast Web View) that a browser can render page-one before downloading the rest, run the merged PDF through the Linearize PDF tool. Linearization reorders objects so that the first page's content, fonts, and hint tables sit at the top of the file, satisfying section 7.5.4 of ISO 32000-2. This is essential for large PDFs delivered via byte-range requests over CDN — bookmark-and-bandwidth heavy academic textbooks are the canonical example.

Can the merged PDF conform to PDF/A or PDF/X archival and print standards?

Merging two PDF/A-1b files does not automatically produce a PDF/A-1b output, because pdf-lib does not enforce the conformance rules (embedded fonts only, no encryption, sRGB or ICC colour, no JavaScript, structured XMP metadata). Run the merged file through a validator like veraPDF, then convert it explicitly with a PDF/A converter or Ghostscript (`-dPDFA=2 -sProcessColorModel=DeviceRGB`). For PDF/X-4 print delivery, your colours must be CMYK or use an embedded ICC profile and every font must be embedded as a subset — again a post-processing step. The cleanest archival workflow is: merge first for content order, then convert once at the end so the conformance flag and DocumentID are written correctly.