I’ve been looking at how online PDF viewers handle text selection and copying. When you select text in these viewers, it seems like you’re just drawing selection boxes over what appears to be an image. But somehow when you press CTRL+C, the actual text gets copied to your clipboard.
I thought this might need Flash to work, but it seems to function even without any plugins installed. What’s the technical approach behind making this text copying feature work in web browsers?
I’m curious about the underlying mechanism that allows users to copy readable text from what looks like just an image file displayed in the browser.
totally, they use layers, like there’s a hidden text over the image. When you highlight, you’re pulling that text out, not just the pic. PDFs are smart like that, keeping text and images apart so browsers can get the text from under the layers.
It works by creating an invisible text layer that matches exactly what you see on screen. When the PDF loads, the viewer pulls the actual text and positioning data from the file. This text gets rendered as hidden HTML elements positioned right over the visual content using CSS. Your browser treats these as regular text, so you can select and copy normally. You think you’re selecting image pixels, but you’re actually grabbing these invisible text elements. The selection highlighting you see is just the browser’s standard text selection at work. Works in all modern browsers without plugins - just uses standard web tech.
It’s all about how PDF.js (Mozilla’s JavaScript PDF renderer) handles the document structure. When a PDF loads, the renderer pulls out the text content and uses CSS transforms to position it exactly over the visual elements. This creates an invisible text layer that sits right where the rendered text appears on screen. You can still select and copy the text because it’s actual DOM text elements - they’re just transparent and positioned correctly. Works without plugins since it’s pure JavaScript and HTML5 canvas rendering. Those selection boxes you see? That’s just normal browser text selection working on these positioned text nodes, not image manipulation.