How PDF Merging Works in Your Browser
This tool uses pdf-lib, a JavaScript library that runs entirely in your browser. When you add PDF files, they are read into memory locally using the File API. The pages from each document are then copied into a new PDF in the order you specify. The final merged document is generated as a binary file and offered for download. At no point does any data leave your device — there is no server involved, no upload, and no cloud storage.
Common Uses for Merging PDFs
Merging PDFs is useful in many everyday and professional scenarios. You might combine multiple scanned pages into a single document, attach supplementary pages to a contract, assemble a multi-chapter report from separate files, or consolidate invoices for an accountant. Estate agents, solicitors, accountants, and administrative teams use PDF merging daily. Having a single coherent document is also easier to share by email, upload to a portal, or store in a filing system.
Reordering Pages Before Merging
The order in which you add files determines the order of pages in the merged PDF. If you need to rearrange, simply drag the file cards into the correct sequence before clicking Merge. This is particularly useful when combining scanned documents that were saved out of order, or when you need a cover page from one file to precede the body from another. Take a moment to preview the intended order before downloading.
File Size and Performance
There is no hard file size limit, but very large PDFs — particularly those with many high-resolution images — can slow down in-browser processing. If you experience slow performance, try working with smaller batches. PDFs containing only text merge very quickly regardless of page count. Once merged, the output file will be approximately the sum of the input file sizes, minus any shared resource deduplication that pdf-lib performs internally.