Why Your Files Should Never Leave Your Browser — Client-Side PDF Processing

Where Does Your PDF Go When You "Upload" It?

You have a PDF that needs a signature. You find an online tool, drag your file onto the page, and sign it. Simple enough. But here is a question most people never ask: where did your file actually go?

With most online PDF tools, the answer is: to someone else's server. Your document — which might contain your home address, Social Security number, salary information, medical records, or confidential business terms — was transmitted over the internet and processed on a computer you do not own or control.

This article explains why that matters, what the alternative looks like, and how to tell the difference.

How Most Online PDF Tools Work (Server-Side Processing)

The traditional architecture for web-based document tools follows this pattern:

You select a file on your device
The file is uploaded to the company's server over an encrypted HTTPS connection
The server processes the file (adds your signature, merges pages, compresses, converts, etc.)
The processed file is sent back to your browser for download
The file may remain on the server for minutes, hours, days, or indefinitely

This is how the majority of online PDF tools work — including many well-known brands. Your file physically leaves your device, travels across the internet, and lands on a server in a data center somewhere.

What Happens to Your File on the Server?

This varies by company and is often opaque. Common scenarios include:

Temporary storage — The file is deleted after processing or after a time window (30 minutes, 24 hours, etc.)
Cached storage — The file is kept for a period to allow re-downloads or for performance optimization
Permanent storage — The file is stored indefinitely, especially if you have an account
Logging and analytics — Metadata about your file (name, size, type, pages) may be logged
Third-party processing — Some tools use cloud services (AWS, Google Cloud, Azure) for processing, meaning your file may pass through multiple systems

Even companies with strong privacy policies cannot eliminate the fundamental risk: your file existed on their infrastructure, which means it was exposed to potential breaches, employee access, government requests, and infrastructure vulnerabilities.

The Alternative: Client-Side Processing

Client-side processing means your file never leaves your device. All the work happens directly in your web browser using JavaScript and WebAssembly.

Here is how it works with SigPDF:

You select a file on your device
Your browser reads the file locally using the File API
JavaScript code running in your browser processes the PDF — parsing it, rendering it, and embedding your signature
The modified file is saved directly from your browser to your device
No network request containing your file is ever made

At no point does the file travel over the internet. The server that hosts SigPDF delivers the web application (HTML, CSS, JavaScript) to your browser, but it never sees your documents. It is architecturally impossible for the server to access your files because they are never transmitted.

Why This Matters: Real-World Scenarios

Contracts and Legal Documents

A contract might contain deal terms, financial figures, proprietary business information, and personal details of the parties involved. Uploading this to a third-party server creates exposure — especially if the contract includes NDAs, non-competes, or sensitive M&A details.

Medical and Healthcare Documents

Patient intake forms, HIPAA authorizations, and medical records contain Protected Health Information (PHI). Under HIPAA, sharing PHI with a third party requires a Business Associate Agreement (BAA). Most free PDF tools do not offer BAAs. Processing these documents client-side eliminates the issue entirely.

Financial Documents

Tax forms, bank statements, loan applications, and pay stubs contain some of the most sensitive personal information: income, Social Security numbers, account numbers, and financial history. Uploading these to a random web tool is a risk that is easily avoided.

Government and Identity Documents

Passports, driver's licenses, visa applications, and government forms often need signatures. These documents contain identity information that is valuable to attackers.

Personal Documents

Rental applications, school enrollment forms, and personal agreements all contain private information that you may not want existing on an unknown server.

How to Tell If a Tool Is Client-Side

Not every tool that claims to be "secure" actually processes files in your browser. Here are ways to verify:

Check the Network Tab

The most definitive test is to open your browser's developer tools (F12 on most browsers), go to the Network tab, and watch what happens when you upload and process a file. If the file is being sent to a server, you will see a large upload request. If processing is client-side, you will see no file upload.

Look for Offline Functionality

Some client-side tools work offline after the initial page load. If you can disconnect from the internet and the tool still processes files, it is running locally. However, not all client-side tools explicitly support offline mode.

Read the Privacy Policy

Look for explicit statements about file processing. Phrases like "your files are processed in your browser" or "we never upload your files" are good signs. Vague statements like "we take your privacy seriously" without specifics are a red flag.

Check the Technology

Client-side PDF processing typically uses JavaScript libraries like pdf-lib (for PDF manipulation), pdf.js (for rendering), or WebAssembly-compiled tools. Server-side processing might reference server technologies, cloud processing, or API endpoints.

SigPDF uses pdf-lib and pdf.js to process PDFs entirely in your browser. You can verify this yourself using the network tab method above.

The Technical Details: How Client-Side PDF Signing Works

For those interested in the technical implementation, here is what happens under the hood when you sign a PDF with a client-side tool:

File reading — The browser's File API reads the PDF bytes into memory as an ArrayBuffer
PDF parsing — A JavaScript library (pdf-lib) parses the PDF structure — pages, fonts, images, annotations, and metadata
Rendering — Another library (pdf.js) renders the PDF pages to HTML5 Canvas elements for visual display
Signature capture — Your signature is drawn on a Canvas element and captured as vector paths or image data
Signature embedding — The signature is embedded into the PDF document structure as an annotation or image on the specified page at the specified coordinates
PDF serialization — The modified PDF is serialized back to bytes
Download — The bytes are wrapped in a Blob and offered as a file download via the browser's download API

The entire process happens in the browser's JavaScript runtime. The server delivers the web app but has no role in the document processing.

What About Encryption in Transit?

Some server-based tools argue that encryption (HTTPS) protects your files during upload. This is true — HTTPS encrypts data in transit, preventing interception by third parties. But it does not address the core issue: the server still receives and processes your unencrypted file.

HTTPS protects data from eavesdroppers on the network. It does not protect data from the server operator, their employees, their subprocessors, attackers who breach the server, or government agencies with legal authority to request data.

Client-side processing makes encryption in transit irrelevant because there is nothing sensitive to transmit.

What About End-to-End Encryption?

Some tools offer end-to-end encryption (E2EE), where files are encrypted on your device before upload and only decrypted after download. This is better than standard server-side processing — the server only sees encrypted data.

However, E2EE still has limitations:

The implementation must be trustworthy (you are trusting the tool's JavaScript to encrypt properly)
Metadata (file name, size, timestamps) may not be encrypted
The server operator could theoretically modify the JavaScript to bypass encryption

Client-side processing is simpler and more transparent: no encryption is needed because no transmission occurs.

The Privacy Spectrum

From least private to most private:

Server-side processing, no encryption at rest — Your files sit on a server in the clear
Server-side processing with encryption at rest — Better, but the server can still decrypt to process
Server-side processing with end-to-end encryption — The server sees only encrypted data, but you trust the implementation
Client-side processing — Your files never leave your device. There is nothing to encrypt, intercept, or breach.

SigPDF operates at level 4. It is the most private architecture possible for a web-based tool.

Build Trust Through Transparency

The beauty of client-side processing is that it is verifiable. You do not need to trust the company's privacy policy — you can inspect the network traffic yourself and confirm that no files are being transmitted. Try it: open SigPDF, open your browser's developer tools, and watch the Network tab while you sign a document. You will see the initial page load, but no file uploads.

That level of transparency is impossible with server-side tools. When a company says "we delete your files after processing," you are trusting their word. When a client-side tool says "your files never leave your browser," you can verify it.

Your Files Deserve to Stay on Your Device

The next time you reach for an online PDF tool, ask yourself: does this file need to leave my device? For signing, the answer is no. SigPDF proves that powerful PDF tools can run entirely in your browser — no uploads, no servers, no risk.

Sign Your PDF Privately →