Free PDF Merger — Combine PDF Files
Combine up to 20 PDFs into a single file, in the order you choose. Free for 2 merges per day; files are processed in memory and never stored.
Why you can't just glue PDF files together
A PDF isn't a flat stream of pages the way a plain text file is a flat stream of characters — it's a structured document with a cross-reference table mapping every object (page, font, image, resource) to a byte offset, plus a trailer pointing to the document's root object. Simply concatenating two PDF files' raw bytes produces a file with two competing cross-reference tables and two trailers, which every standards-compliant reader will refuse to open correctly. A real merge has to walk each source document's object graph, renumber every object so nothing collides, and rebuild one coherent cross-reference table and trailer for the combined result — which is what this tool does using the same PDF library that powers the site's other PDF tools.
One side effect worth knowing: each page keeps its own original page size in the merge. If your source files mix, say, US Letter and A4 pages, the combined PDF will too — most PDF viewers handle that without complaint, but it's worth checking before printing a merged document that spans multiple original page sizes.
Frequently Asked Questions
In what order are the PDFs combined?
Top to bottom as shown in the list. Use the ↑ and ↓ buttons next to each file to reorder before merging.
Are my PDFs uploaded and stored?
Files are sent to our server for merging, processed entirely in memory, streamed back as your download, and immediately discarded. Nothing is written to disk or retained.
Why is there a limit of 2 free merges per day?
Server-side PDF processing costs real resources. A one-time $2.99 unlock removes the daily limit on all four PDF tools permanently — no subscription.
Can I merge password-protected PDFs?
No — remove the password first (most PDF viewers can save an unlocked copy when you know the password). Protected files are rejected with a clear error rather than producing a broken merge.
Why can't I just concatenate the PDF files myself?
Every PDF has its own internal cross-reference table and trailer pointing to its object structure. Pasting two files' bytes together produces two competing tables in one file, which readers can't reconcile — a proper merge renumbers and relinks every internal object into one coherent structure.
What happens if my source PDFs have different page sizes?
Each page keeps its original size in the merged output — nothing is stretched or forced onto a common page size. A merge combining US Letter and A4 documents, for example, will still contain pages of both sizes.