--- title: How to Build a Browser-Based PDF Redaction Tool Using... description: A step-by-step guide to building a browser-based PDF redaction tool using JavaScript, PDF.js, and PDF-lib. Covers the full workflow: uploading a PDF, rendering... canonical: https://daily.dev/posts/how-to-build-a-browser-based-pdf-redaction-tool-using-javascript-jti1xfs3t twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: How to Build a Browser-Based PDF Redaction Tool Using JavaScript | daily.dev og:description: A step-by-step guide to building a browser-based PDF redaction tool using JavaScript, PDF.js, and PDF-lib. Covers the full workflow: uploading a PDF, rendering... og:url: https://daily.dev/posts/how-to-build-a-browser-based-pdf-redaction-tool-using-javascript-jti1xfs3t og:image: https://api.daily.dev/og/posts/JTI1XFS3T.png og:image:alt: How to Build a Browser-Based PDF Redaction Tool Using JavaScript og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # How to Build a Browser-Based PDF Redaction Tool Using JavaScript **[freeCodeCamp](https://daily.dev/sources/freecodecamp)** · [@allinonetools](https://daily.dev/allinonetools) · 23 min read · 1 upvotes · 1 comments ## Summary A step-by-step guide to building a browser-based PDF redaction tool using JavaScript, PDF.js, and PDF-lib. Covers the full workflow: uploading a PDF, rendering pages on a canvas, drawing redaction rectangles via pointer events, storing coordinates per page, converting canvas coordinates to PDF coordinates (including Y-axis origin differences), applying redactions to single or multiple pages, generating the modified PDF, previewing the result, and downloading it. Also addresses the critical security distinction between visually masking content with an opaque rectangle versus truly removing underlying PDF content, plus performance tips for handling large documents. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How do I convert canvas coordinates to PDF coordinates when drawing redaction boxes with PDF.js and PDF-lib? Calculate scaleX and scaleY as the ratio of the PDF page's actual width/height to the canvas width/height, then multiply the box's x, y, width, and height by those scale factors. Because canvas coordinates start at the top-left while PDF coordinates start at the bottom-left, the Y value must also be flipped using pdfHeight minus (redaction.y plus redaction.height) times scaleY. _Developers wiring up canvas-to-PDF coordinate math can find implementation walkthroughs like this one through daily.dev._ ### Does covering text with a black rectangle in a PDF actually remove the underlying content? No, drawing an opaque rectangle over text or an image in a PDF only visually hides it; the underlying text, image, or annotation content can still remain in the PDF structure and may be selectable, searchable, or recoverable. True redaction for legally sensitive or compliance-critical documents requires a standards-based process that removes the underlying content objects and sanitizes metadata, not just visual masking. _Anyone building document redaction features can weigh this security nuance via related coverage on daily.dev._ ### Which JavaScript libraries can I use to build a PDF redaction tool that runs entirely in the browser without a backend? PDF.js can load and render PDF pages onto an HTML canvas so users can preview and draw over them, while PDF-lib can open the original PDF, apply drawn rectangles to selected pages, and generate a new processed PDF file, all without sending the document to a server, keeping processing local for privacy. _Teams comparing client-side PDF processing libraries can track options like this through daily.dev._ ## Community discussion Top comments from developers on daily.dev. **@allinonetools** · 1 upvotes > One thing that surprised me while building this was how tricky PDF coordinate mapping is. ## Similar posts on daily.dev - [PDF Redaction in JavaScript: A Practical Guide to Protecting Sensitive Data](https://daily.dev/posts/pdf-redaction-in-javascript-a-practical-guide-to-protecting-sensitive-data-8pqw16b72) · Syncfusion · 0 upvotes · 0 comments - [How to Build a Browser-Based PDF to Image Converter Using JavaScript](https://daily.dev/posts/how-to-build-a-browser-based-pdf-to-image-converter-using-javascript-ixeedosyr) · freeCodeCamp · 12 upvotes · 0 comments - [How to Build a Browser-Based PDF Resizer Using JavaScript](https://daily.dev/posts/how-to-build-a-browser-based-pdf-resizer-using-javascript-aqogolius) · freeCodeCamp · 7 upvotes · 0 comments - [JavaScript PDF Editor for Web Apps: Advanced PDF Editing Made Easy](https://daily.dev/posts/javascript-pdf-editor-for-web-apps-advanced-pdf-editing-made-easy-k86ikliln) · Syncfusion · 5 upvotes · 0 comments - [How to Build a Browser-Based PDF OCR to Text Converter Using JavaScript](https://daily.dev/posts/how-to-build-a-browser-based-pdf-ocr-to-text-converter-using-javascript-h9luff8eo) · freeCodeCamp · 3 upvotes · 2 comments --- Tags: [#javascript](https://daily.dev/tags/javascript) [View this post on daily.dev](https://daily.dev/posts/how-to-build-a-browser-based-pdf-redaction-tool-using-javascript-jti1xfs3t) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"How to Build a Browser-Based PDF Redaction Tool Using JavaScript","url":"https://daily.dev/posts/how-to-build-a-browser-based-pdf-redaction-tool-using-javascript-jti1xfs3t","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/how-to-build-a-browser-based-pdf-redaction-tool-using-javascript-jti1xfs3t"},"datePublished":"2026-07-17T23:08:41.149Z","dateModified":"2026-09-14T09:00:41.968Z","description":"A step-by-step guide to building a browser-based PDF redaction tool using JavaScript, PDF.js, and PDF-lib. 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