Merge PDF Files

A PDF file is, in spec terms, a body of numbered objects (pages, fonts, images, content streams) plus a cross-reference table that tells a reader the byte offset of each object. The page tree (ISO 32000-2 §7.7.3.2) is a shallow structure: a root /Pages dictionary points to /Kids (each a /Page object), and the catalog points to that root. To merge two PDFs you copy every page object from each source — together with everything it references (resources, fonts, images, content streams) — into a fresh document and rebuild a single page-tree root over the lot. That is what this tool does.

The merge is performed by pdf-lib, an MIT-licensed pure-JavaScript PDF library. It is loaded dynamically when you press Merge — until then the page bundle stays small and your browser does not download the ~270 KB library. Inside the React component the flow is straight: PDFDocument.create() for the destination, PDFDocument.load(bytes) for each source, then copyPages(src, indices).addPage(...) for every page, and finally save() to serialise.

For each PDF the tool does five deterministic things before the merge:

1. Validate the file. Confirms the PDF type, a non-zero size, and a per-file cap of 100.0 MB. The first 1 KB is scanned for the literal %PDF-n.m header (ISO 32000-2 §7.5.2), and the last 2 KB for the %%EOF terminator (§7.5.5). Files missing either are rejected with an explanation.
2. Read the version. The two ASCII digits inside the header give the PDF version (1.4 through 2.0 in real-world files). The version is shown on each queued row so you can spot a 2.0 file mixed with a 1.4 file before pressing Merge.
3. Estimate page count. A heuristic byte-stream scan counts occurrences of /Type /Page (excluding /Pages). This is fast and accurate on every uncompressed PDF and on most modern PDFs. PDFs that store every page object inside a compressed object stream (ISO 32000-2 §7.5.8) under-count here — the exact count is reported after the merge.
4. Order the queue. Files are merged in the order shown. Use the up and down arrows to nudge a file, or the trash icon to drop it. The page order in the output PDF mirrors the queue exactly.
5. Merge and serialise. On Merge, pdf-lib creates a destination document, copies every page from each source (preserving the original byte streams), assigns them to the new page tree, writes a fresh catalog and trailer, and returns the merged byte buffer. That buffer is wrapped in a Blob and offered to you as a download via an object URL. No bytes leave the browser tab.

• ISO 32000-2 — PDF 2.0 specification (file header §7.5.2, trailer §7.5.5, page tree §7.7.3.2, object streams §7.5.8)
• pdf-lib — MIT-licensed pure-JavaScript PDF library used for the page-tree merge
• pdf-lib source on GitHub (open-source, audited)
• MDN — File API (used for in-browser file reads, no upload)

The PDF header and trailer validators were cross-checked against ISO 32000-2 on 2026-05-11. The pdf-lib dependency is pinned in package.json and re-verified on each major-version bump.