How do I determine the size of a pdf with pdf.js so I can scale to the screen size?

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07-10-2024

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Sizing Up Your PDFs: Mastering PDF.js for Dynamic Scaling

The Problem: You've got a PDF, and you want to display it perfectly on a user's screen, regardless of its size. This means adjusting the PDF's display to fit the browser window. But how do you determine the PDF's actual dimensions to calculate the appropriate scaling factor?

The Solution: PDF.js, a JavaScript library for rendering PDFs in the browser, offers the perfect solution. Here's how to harness its power to dynamically scale your PDFs:

Understanding the Code:

Let's look at a basic example of loading a PDF into the browser and obtaining its dimensions:

const canvas = document.getElementById('myCanvas');
const context = canvas.getContext('2d');

PDFJS.getDocument('your_pdf.pdf').then(pdf => {
  const numPages = pdf.numPages;
  
  pdf.getPage(1).then(page => {
    const viewport = page.getViewport({scale: 1}); 
    canvas.width = viewport.width;
    canvas.height = viewport.height;
  });
});

Breaking Down the Code:

1. Loading the PDF: PDFJS.getDocument('your_pdf.pdf') retrieves the PDF file.
2. Getting Page Information: pdf.getPage(1) gets the first page (you can change 1 to access other pages).
3. Calculating Viewport: page.getViewport({scale: 1}) creates a viewport object, representing the PDF page at a scale of 1 (meaning 1 pixel in the PDF is 1 pixel on the screen).
4. Setting Canvas Size: canvas.width and canvas.height set the dimensions of your canvas to match the PDF page.

Beyond the Basics:

• Dynamic Scaling: You can easily adjust the scale parameter in the getViewport method to control how large or small the PDF appears on the screen.
• Fitting the Screen: To make the PDF fit the screen, calculate the desired scale factor based on the screen width and height and the PDF dimensions. For example, you could divide the screen width by the PDF width to get the appropriate scale for a horizontal fit.
• Multiple Page Support: Iterate through all the pages in the PDF, calculate their viewports, and render them onto separate canvases for seamless multi-page display.

Additional Tips:

• Responsive Design: Combine the dynamic scaling technique with CSS media queries to create a truly responsive PDF viewer, adapting to different screen sizes automatically.
• Performance Optimization: For large PDFs, consider techniques like lazy loading of pages or using workers to offload the PDF rendering process.
• User Experience: Provide user controls like zoom buttons, a page navigation bar, and search functionality to enhance interactivity and user experience.

References and Resources:

• PDF.js Documentation: Comprehensive documentation for the PDF.js library.
• PDF.js Examples: Explore various use cases and code examples.
• W3Schools - JavaScript Canvas: A good resource to learn about the <canvas> element.

By understanding the fundamentals of PDF.js and implementing dynamic scaling, you can create a seamless and user-friendly experience for viewing PDFs on any device.