For a long time now, anyone googling "merge PDF" and clicking the first link to merge PDFs has been uploading their files to some unknown server of a company they've never even heard of. While this can be annoying (honestly, it's an unnecessary step) for normal files, it poses a real risk to your privacy for contracts, payslips, medical records, and tax documents.
Instead, this guide outlines what happens when you upload PDFs to free online tools and explains how to merge files locally in the browser of your choice.
Merge PDF files — runs entirely in your browser →
Why uploading PDFs is riskier than most people think
Server retention
File stored for hours — sometimes archived indefinitely.
OCR indexing
Names, addresses, account numbers extracted as searchable text.
Metadata extraction
Author, company, "Confidential" flag — pulled out and stored.
GDPR & jurisdiction
Hosted abroad under weaker data-protection laws.
Four things that happen the moment your PDF hits a third-party server.
Free PDF tools online are powered by a pretty obscure business model. Although the tools are free to use, the companies providing them earn money in ways unrelated to charging users for the tools. Understanding these business models can also change your view of which tools are best to use.
Server retention. Most free online PDF tools upload files to cloud servers and then delete them after a certain time span. Usually, this is after one hour. But in reality, some services might keep your files for an indefinite time or even store archived versions of them. As these policies are usually hidden in the privacy policy, they rarely get read by users.
OCR indexing. The same free PDF services that offer to process your PDF files for free to make searchable documents (e.g., a list of names and addresses) within the PDFs also use the extracted text (e.g., names, addresses, account numbers, contractual language and terms and conditions) to index the processed PDFs on their servers.
Metadata extraction. PDFs contain a large amount of metadata (author, company, creator application, modification date, document title, etc.). Some services extract and store this metadata as separate data while processing the PDF. A document marked as "Confidential" in the metadata is not confidential after it has been processed and indexed by a third-party server.
GDPR and jurisdiction. Many free PDF services are operated from countries with very poor data protection compared to EU or UK laws. If you are subject to the provisions of the GDPR (you process information about EU citizens), uploading client documents to free online services in unknown jurisdictions could well be against the law.
I'm not suggesting that there is anything wrong with all online PDF tools. Most of them are pretty simple utilities written by pretty honest people. But you have no way of knowing for sure, and most services are hosted in countries with less stringent data protection laws than in the EU or even in the UK, for instance. All of which can be a problem when GDPR applies (i.e., when you're processing information about EU citizens).
What in-browser PDF merging actually means
Given that ConvertYard runs in-browser, it uses WebAssembly (WASM) to run the tool code in the same JavaScript runtime that runs the rest of the web page, on your computer. This means that the PDF library (pdf-lib) runs locally on your computer as well.
As opposed to normal web applications, no file data is uploaded to servers in "in-browser" processing with WebAssembly. First, a PDF is read by the browser's FileReader API into the memory of the machine you are running on. This memory is then manipulated by the WASM library. The output is written to the browser's memory and then offered for download on the browser's file system. Only the initial page load of the tool was transmitted over the network.
You can verify this by opening the browser's developer tools, switching to the Network tab, and then merging a file. The tool should not send any POST requests containing the file data in this case.
Step-by-step: merge PDFs locally in your browser
- Go to ConvertYard's PDF merger.
- Add PDF files to the dropzone above or click on the files to select them.
- Arrange your file cards in the desired order for your merged document.
- Click Merge.
- Download the merged PDF. The file size of the merged file is approximately the sum of the sizes of the input files (sometimes slightly smaller because of deduplication of the same resources like fonts).
Also note that even if the merged file is too large for sending by e-mail or uploading to a web server, it can still be reduced in size by ConvertYard's Compress PDF tool without impairing its look.
Merging 50+ PDFs in order
For bigger merges (like monthly reports, archives of document sets, or multi-chapter documents), the drag-to-reorder functionality of ConvertYard is crucial. Simply drag and drop the file cards in the desired order before you start to merge.
Tips for large merges:
- Upload files in the correct order and name them in sequence (e.g.,
01-january.pdf,02-february.pdf, etc.). As mentioned above, the uploaded files are sorted alphabetically. - Be aware of total file size. Large file batches (over 500 MB) might consume too much memory on older hardware. Try to split up large batches first and then merge the results.
- Page size consistency. PDFs of different page sizes can be merged together. The converted merged PDF will contain each page at its original size (A4, Letter, etc.).
When you might still need a desktop app
For the vast majority of use cases, in-browser PDF merging will suffice. However, there are several scenarios in which you will be better off using desktop software instead.
Password-protected PDFs: in-browser PDF merging tools can't handle such PDFs unless you provide the password. This typically means first removing the open-password protection in a suitable desktop app or using ConvertYard's own Unlock PDF tool.
Very large documents in bulk. As an example, merging 500+ PDFs totaling several GB can push the limits of available browser memory. Here, large collections of PDFs can be merged much more easily with tools like PDF24 in offline mode or even with command-line tools like pdftk.
Advanced features like preserving bookmarks, adding page labels, and flattening forms across all merged documents are currently not fully supported by in-browser tools. In case you are dealing with complex document workflows, you might even consider using Adobe Acrobat or a full document management system.
However, simple merging of documents like invoices, reports, or scanned pages can easily be done with an in-browser tool without the overhead of a subscription or a local installation.