How PDFlys Keeps Your Files Private — The Technical Details

The Trust Problem with Online PDF Tools

When you upload a sensitive PDF — a contract, tax return, medical record, or legal document — you deserve to know exactly what happens to it. Most online PDF tools upload your files to their servers, process them remotely, and promise to "delete them later." You're trusting a privacy policy instead of verifiable architecture.

PDFlys eliminates that trust problem entirely. Your files are processed in your browser and never transmitted anywhere. This post explains exactly how that works — technically, not just as a marketing claim.

Verify It Yourself in 30 Seconds

This is the single most important section of this post. You don't need to trust our word — you can watch the proof in real time.

1. Open any PDFlys tool (e.g., Merge PDF)
2. Open your browser's Developer Tools (F12 or Ctrl+Shift+I)
3. Click the Network tab
4. Upload a PDF and process it
5. Watch the Network tab — you'll see zero upload requests to PDFlys servers

Your file stays in your browser's memory from upload to download. The network log is the proof.

How Browser-Based Processing Works

Here's the technical flow for every PDFlys operation:

Step 1: You Select a Tool

Choose any of the 28 PDF tools — merge, split, compress, sign, redact, add page numbers, check accessibility, and more.

Step 2: Your File Loads Into Browser Memory

When you select a PDF, your browser reads it into a JavaScript ArrayBuffer in memory. No network request is made. The file object exists only in your browser's process.

Step 3: JavaScript Libraries Process the File

PDFlys uses JavaScript-based PDF libraries — primarily pdf-lib for document manipulation and PDF.js for rendering and analysis. These libraries run entirely in your browser's JavaScript engine. Your CPU does the work, not a remote server.

Step 4: You Download the Result

The processed file is generated as a Blob in browser memory. When you click download, your browser writes it to disk directly. PDFlys never sees the output.

What PDFlys Does and Doesn't Collect

Does NOT collect:

• Your PDF files (never transmitted)
• File names, contents, or metadata
• Signature data, form field values, or redacted content
• Any information from files you process

Does collect (minimal, anonymized):

• Page views and button clicks via Plausible Analytics — a cookie-free, privacy-first analytics tool
• JavaScript error reports via Sentry — for fixing bugs. Error reports never contain file data

Both services are disclosed on the Security & Trust page with full technical details.

Why This Architecture Matters for Sensitive Documents

Client-side processing isn't just a privacy feature — it's a security architecture. When your files never leave your device:

• Data breaches can't expose your files — there's nothing on the server to breach
• No employee access — PDFlys staff can't view, copy, or retain your documents
• No retention policy to worry about — files aren't stored, so there's nothing to "delete after 24 hours"
• Air-gapped compatibility — because processing is local, PDFlys works even on restricted networks (after the initial page load)

This makes PDFlys suitable for contracts, healthcare records, legal filings, financial documents, and any other sensitive PDFs.

The Security & Trust Page

All of the above is documented in permanent detail on the Security & Trust page, accessible from the site footer. It covers:

• Zero-upload architecture with technical explanation
• Data storage practices (none)
• Network security details
• Third-party service disclosures
• Code transparency (open-source core libraries)
• User control and data lifecycle

The page exists for users who want to verify our claims, compliance teams running vendor assessments, and anyone who needs technical documentation before trusting a tool with sensitive files.

Frequently Asked Questions

How is this different from tools that say "we delete your files after processing"?

Those tools still upload your files to a server — your data exists on their infrastructure, even temporarily. PDFlys never receives your files in the first place. There's nothing to delete because nothing was transmitted.

Can my employer's IT team verify the no-upload claim?

Yes. Any network monitoring tool (browser DevTools, Wireshark, corporate proxy logs) will confirm that no file data is sent to PDFlys servers during processing. The Security & Trust page provides documentation suitable for IT vendor assessments.

What happens if my browser crashes during processing?

The file in browser memory is lost — you'd re-upload and try again. But this also means there's no server-side copy that persists. Crashed sessions leave no trace.

Does client-side processing limit what PDFlys can do?

Modern JavaScript engines are powerful enough to handle complex PDF operations (merging, splitting, compressing, signing, redacting) on documents up to hundreds of pages. For extremely large files, you may notice longer processing times since your device's CPU is doing the work.

Are the PDF libraries open source?

Yes. pdf-lib and PDF.js are both open-source projects with public GitHub repositories. You can audit the code that processes your files.

Where to Go from Here

If privacy is a priority for your document workflow, these tools pair well with the security architecture described above:

• Protect PDF — Add password encryption to documents before sharing
• Redact PDF — Permanently remove sensitive content (see our detailed redaction guide)

For the full technical breakdown, visit the Security & Trust page. For everything else, the DevTools Network tab is your proof.