Annotations in PDFKit

Annotations are interactive features of the PDF format, and they make it possible to include things like links and attached notes, or to highlight, underline or strikeout portions of text. Annotations are added using the various helper methods, and each type of annotation is defined by a rectangle and some other properties. Here is a list of the available annotation methods:

• note(x, y, width, height, contents, options)
• link(x, y, width, height, url, options)
• goTo(x, y, w, h, name, options)
• highlight(x, y, width, height, options)
• underline(x, y, width, height, options)
• strike(x, y, width, height, options)
• lineAnnotation(x1, y1, x2, y2, options)
• rectAnnotation(x, y, width, height, options)
• ellipseAnnotation(x, y, width, height, options)
• textAnnotation(x, y, width, height, text, options)
• fileAnnotation(x, y, width, height, file, options)

Many of the annotations have a color option that you can specify. You can use an array of RGB values, a hex color, or a named CSS color value for that option.

If you are adding an annotation to a piece of text, such as a link or underline, you will need to know the width and height of the text in order to create the required rectangle for the annotation. There are two methods that you can use to do that. To get the width of any piece of text in the current font, just call the widthOfString method with the string you want to measure. To get the line height in the current font, just call the currentLineHeight method.

You must remember that annotations have a stacking order. If you are putting more than one annotation on a single area and one of those annotations is a link, make sure that the link is the last one you add, otherwise it will be covered by another annotation and the user won't be able to click it.

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Here is an example that uses a few of the annotation types.

// Add the link text
doc.fontSize(25)
   .fillColor('blue')
   .text('This is a link!', 20, 0);

// Measure the text
const width = doc.widthOfString('This is a link!');
const height = doc.currentLineHeight();

// Add the underline and link annotations
doc.underline(20, 0, width, height, {color: 'blue'})
   .link(20, 0, width, height, 'http://google.com/');

// Create the highlighted text
doc.moveDown()
   .fillColor('black')
   .highlight(20, doc.y, doc.widthOfString('This text is highlighted!'), height)
   .text('This text is highlighted!');

// Create the crossed out text
doc.moveDown()
   .strike(20, doc.y, doc.widthOfString('STRIKE!'), height)
   .text('STRIKE!');

// Adding go to as annotation
doc.goTo(20, doc.y, 10, 20, 'LINK', {});

The output of this example looks like this.

Annotations are currently not the easiest things to add to PDF documents, but that is the fault of the PDF spec itself. Calculating a rectangle manually isn't fun, but PDFKit makes it easier for a few common annotations applied to text, including links, underlines, and strikes. Here's an example showing two of them:

doc.fontSize(20)
   .fillColor('red')
   .text('Another link!', 20, 0, {
     link: 'http://apple.com/',
     underline: true
   }
);

The output is as you'd expect: