如何使用IText7和C＃在现有PDF中为内部链接加下划线？

Question

We regularly use Word to generate large documents with many internal cross-references. I'm writing a tool to find and add blue underlining to these clickable internal cross-references. (I know I can do this at the Word level, but I need to be able to do this even when I don't have access to the original Word file.)

I am using C# and IText 7. I am able to load and scan the existing PDF for these internal links. But what I can't seem to do is change the links' appearance. After searching, I learned that the PdfAnnotation class is for creating new links and not for altering existing ones. I'm told I need to use Put() to change existing ones. I've tried a number of different approaches, but none of them seem to be working.

var pdfDoc = new PdfDocument(new PdfReader(txt_FileLoaded.Text), new PdfWriter(outfile));
for (int x = pgStart; x <= pgEnd; x++)
{
    PdfPage page = pdfDoc.GetPage(x);
    var annotations = page.GetAnnotations();
    foreach (var a in annotations)
    {
        if (a.GetSubtype() == PdfName.Link)
        {
            PdfLinkAnnotation link = (PdfLinkAnnotation)a;
            var action = link.GetAction();
            if (action != null)
            {
                if ( 
                    (action.Get(PdfName.S) == PdfName.URI) || 
                    (action.Get(PdfName.S) == PdfName.GoToR) )
                {
                    //Do something with external links if you want
                }
                else if ( 
                    (action.Get(PdfName.S) == PdfName.GoTo) ||
                    (action.Get(PdfName.S) == PdfName.GoToE))
                {
                    //Do something with internal links
                    link.Put(PdfName.C, new PdfArray(ColorConstants.BLUE.GetColorValue()));
                    link.Put(PdfName.Border, new PdfArray(new int[] {0,0,5}));
                }
            }
        }
    }
}
pdfDoc.Close();

The new file is correctly written when I call Close() , but no underlining is appearing. Again, I've confirmed that it's indeed finding the links. What specific changes do I need to make via Put() to add these underlines?

Thanks for your time!!

Answer 1

To the best of my knowledge what you are trying to achieve cannot be achieved by purely modifying the annotation objects. It might be possible to try playing with appearance streams but I'm not sure it will work and even if it will it can have some side effects.

Annotations just encode the rectangular area which becomes clickable and the action (or destination) that will be triggered when that area is clicked. It is also possible to configure the border of the annotation but PDF specification does not allow you to do fine-grained tuning and thus you won't be able to just set the bottom border to emulate the underlining.

What we will be doing instead is drawing a line directly in the contents of the page in the hope that the annotation position is accurate enough.

We can get annotation's area with annotation.getRectangle() , and then use PdfCanvas to draw a line with matching coordinates (you could also shift it upwards a bit if needed). The code is in Java, but you will find it very easy to convert to C# since only the method names would start with the capital letters.

PdfPage page = pdfDocument.getPage(i);
// Create canvas where we would draw additional lines
PdfCanvas pageCanvas = new PdfCanvas(page);
for (PdfAnnotation annotation : page.getAnnotations()) {
    Rectangle annotationArea = annotation.getRectangle().toRectangle();
    // Draw a line at the bottom of the annotation area
    pageCanvas.setStrokeColor(ColorConstants.BLUE).
            moveTo(annotationArea.getLeft(), annotationArea.getBottom()).
            lineTo(annotationArea.getRight(), annotationArea.getBottom()).
            stroke();
}

This is how the initial PDF looks like:

This is how the output PDF looks like