Merging PDFs is one of the most common document tasks: combining signed contracts, assembling visa applications, putting together a portfolio, or stitching invoice pages into a single file. It is also a task that routinely involves sensitive material — tax records, medical paperwork, identity documents, legal correspondence. Every time you merge these files on a server-side tool, you are handing them to a third party. This guide explains why client-side merging is safer and how to do it with FormFix.
The risk with server-side PDF mergers
Most “free online PDF merger” services work the same way: you upload your files, the server stitches them together, and you download the result. What happens to the originals in between varies. Some services delete files after 30 minutes; some keep them for 30 days; some log metadata indefinitely; some have been caught selling anonymised document content to analytics and AI training pipelines. The privacy policy will usually say “we may retain files for service improvement” — wording broad enough to cover almost anything.
The risk is not theoretical. Law firms, healthcare providers, and financial institutions have all suffered breaches originating in third-party document-processing tools. And even when nothing bad happens, the metadata trail — IP address, timestamp, file names, page counts — is permanent and outside your control.
How client-side merging works
FormFix’s Merge PDF tool runs entirely in your browser. The flow is:
- Select files. Drag and drop two or more PDFs onto the upload zone, or click to browse.
- Reorder. Drag the files into the order you want them to appear in the output.
- Merge. Click the button. The browser reads each file into memory, parses the page trees with
@libpdf/core, copies the page streams into a single new PDF document, and serialises the result as aUint8Array. - Download. The merged file is presented as a download from a local
Blob. No bytes are uploaded.
Because @libpdf/core operates on the parsed PDF structure rather than re-rendering pages, the merge is lossless — fonts, vector graphics, embedded images, hyperlinks, and form fields all come through unchanged. The output is typically byte-comparable to what a desktop PDF editor would produce.
What about performance?
Merging 10 typical PDFs (say, 20 pages each, a few hundred KB per file) takes under a second on a modern device. Large files (100+ MB) take longer, mainly because of disk I/O and parsing, but the work happens in a Web Worker so the page stays responsive. There is no upload bottleneck, no queue, no rate limit — the bottleneck is your own CPU, which is usually faster than a round-trip to a cloud service.
Cancellation and cleanup
You can cancel a merge in progress by clicking the cancel button or closing the tab. The browser reclaims the memory immediately. There are no partial uploads to clean up on someone else’s server, no failed jobs sitting in a queue waiting to be retried, no temporary files lingering on disk in a data center.
When to use this approach
Client-side merging is ideal for:
- Confidential documents — contracts, NDA paperwork, financial statements.
- Identity documents — passport scans, visa applications, government forms.
- Healthcare records — anything covered by HIPAA, GDPR, or similar regulation.
- Offline work — when you have no internet connection, the tool still works.
- Bulk merging — no per-file upload time means you can combine dozens of PDFs in seconds.
For tasks that involve editing the contents of a PDF (changing text, replacing images), a desktop tool like Adobe Acrobat or LibreOffice Draw is still the better choice. For pure structural operations — merge, split, rotate, reorder — the browser is now the safest and often the fastest option.
Verifying privacy
You do not need to take our word for it. Open your browser’s DevTools, go to the Network tab, and run a merge. You will see requests for the page itself, the JavaScript chunks, and the favicon — and nothing else. No file traffic, no uploads, no analytics on document content. The only request that contains any of your data is the final download, which is between your browser and your own disk.
Try the Merge PDF tool now — and check the Network tab while you do.