Web tools that edit PDFs almost always work by uploading your file to
a server, processing it there and sending it back. That means your
document sits on someone else's computer, at least briefly, and you
are trusting their retention policy, their staff and their security.
This tool has no server component at all.
What actually happens when you open a file
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Your browser downloads the application once — some HTML, JavaScript
and CSS. That is the only network request involved in the whole
process.
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Choosing a file hands the page a reference to it through the
browser's own file-picker. The bytes are read into memory with the
standard
File.arrayBuffer() call — the same mechanism
an image viewer uses.
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pdf.js parses the document. It runs on a Web
Worker, a second thread inside your browser, so a large file does
not freeze the interface. Pages are drawn onto a
<canvas> element to produce what you see.
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Edits change a plain data structure in memory — which page goes
where, what is written on it. Your original file is never modified.
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pdf-lib assembles a new PDF as raw bytes, still in
memory.
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Saving wraps those bytes in a
Blob and hands them to
the browser's download mechanism via an
object URL — an address that exists only inside this
tab and is meaningless anywhere else. No request is made.
How you can check this yourself
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Turn off your internet after the page loads. Every
feature keeps working. Nothing that talks to a server could.
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Open your browser's developer tools and watch the Network
tab. Load a PDF, edit it, save it. You will see no requests
beyond the initial page load.
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Read the source. The whole thing is public at
github.com/lakshyathukral/pdf-tool.
What is stored
Only your settings — watermark text, numbering format, saved presets —
kept in this browser's local storage on this device. No document, no
page, no filename. Clearing your browser data removes it. There are no
analytics, no tracking, no cookies and no error reporting.
What this does not protect against
Being honest about the limits, because a security claim that overstates
itself is worse than none:
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It does not protect against a compromised computer, a malicious
browser extension, or someone with access to your screen. Anything
running on your device can see what your browser is doing.
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Once you save a file, ordinary care applies — this tool has no say
in where it goes next.
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Redaction removes text by rendering the page to an image, which is
genuinely irreversible, but it does not remove attachments or every
trace of document structure. Check important redactions before
sending.
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A signature image or a watermark is a picture. Neither is a
cryptographic signature, and neither makes a document tamper-proof.