Modifying a PDF Document

In this tutorial you will learn how to open an existing PDF document and modify it by adding pages and changing existing pages. If you didn’t do the previous tutorial, grab this PDF file and save it in your working directory.

Opening an Existing Document

Similar to File.open HexaPDF also provides an #open method to easily open local files:

require 'hexapdf'

doc = HexaPDF::Document.open('hello-world.pdf')

The variable doc now contains a HexaPDF::Document instance with the data from hello-world.pdf.

Drawing onto an Existing Page

Since PDF is a complex file format, HexaPDF provides convenience methods so that looking at the PDF specification is not necessary for many things. One of these methods is HexaPDF::Document#pages that allows you to access existing pages as well as add or delete them. Now we get the first page:

page1 = doc.pages[0]

The page class also provides convenience methods. We alrady uesd the #canvas method in the previous tutorial. Now we use it again to draw beneath and atop the existing page:

underlay_canvas = page1.canvas(type: :underlay)
underlay_canvas.fill_color(50).
  rectangle(0, 0, page1.box.width, page1.box.height).
  fill

This draws a gray background. Note that this will only work if the page doesn’t already have a background (as is the case with our sample PDF) because later drawing operations always draw over any existing drawings.

We utilize this drawing order to also draw something atop the page:

overlay_canvas = page1.canvas(type: :overlay)
overlay_canvas.fill_color(129, 192, 255).
  font('Helvetica', size: 50).
  save_graphics_state.
  translate(170, 330).rotate(30).
  text("Hello World", at: [0, 0]).
  restore_graphics_state.
  translate(170, 470).rotate(-30).
  text("Hello World", at: [0, 0])

This code uses several methods of the canvas object to draw rotated versions of the text “Hello World”. Note how we first translate and then rotate the canvas before drawing the text at the – new – origin.

Re-Using Content on Multiple Pages

The PDF format has a mechanism for using the same content on multiple pages called “Form XObjects”. Note that these form XObjects have nothing to do with interactive PDF forms!

We will use such a form XObject to create the background layout of the content pages. First we create the needed form XObject and get its canvas object:

form = doc.add(Type: :XObject, Subtype: :Form, BBox: HexaPDF::Type::Page.media_box(:A4))
form_canvas = form.canvas

In this case we create the form XObject directly by specifying all the properties; the returned object provides the appropriate convenience methods. Since the form XObject has the same size as the document’s pages, we don’t have to scale it later.

Then we will use the canvas object to draw our background layout:

form_canvas.
  fill_color(0, 102, 204).
  rectangle(0, 0, form.width, 50).
  rectangle(0, form.height - 50, form.width, 50).
  fill.
  fill_color(255).
  font('Helvetica', size: 30, style: :italic).
  text('Tutorial 2', at: [15, form.height - 35])

Note how we chained two drawing operations before calling #fill.

Now that our form XObject is ready we use the underlay canvas to add it to the existing page:

underlay_canvas.xobject(form, at: [0, 0])

Inserting New Pages

Now that we have updated our existing page, it is time to add new pages. To insert a page at a specific (zero-based) page index, just use the doc.pages.insert method:

front_page = doc.pages.insert(0)
front_page.canvas.
  font('Helvetica', variant: :bold, size: 50).
  text("Tutorial 2", at: [100, 600]).
  font_size(30).text("Modifying a PDF Document", at: [100, 550]).
  fill_color(0, 102, 204).
  rectangle(0, front_page.box.height - 50, front_page.box.width, 50).
  rectangle(0, 0, front_page.box.width, 50).
  fill

This code inserts a title page as first page of the PDF document. Let’s add some more pages with some simple content:

1.upto(10) do |number|
  canvas = doc.pages.add.canvas
  canvas.xobject(form, at: [0, 0])
  canvas.translate(70, 700).rotate(-55).
    fill_color(0, 128, 255).
    font('Helvetica', size: 90).
    text("Sample Page #{number}", at: [0, 0])
end

Writing the PDF and Experiments

At last, we write the PDF document to a file:

doc.write('modified.pdf')

If you have followed the tutorial, you can now run the program to get the modified PDF.

That is all for now but for further experiments you might wanna try:

The Complete Code and Result PDF

Here is the complete code generating this result PDF:

require 'hexapdf'

doc = HexaPDF::Document.open('hello-world.pdf')

page1 = doc.pages[0]

underlay_canvas = page1.canvas(type: :underlay)
underlay_canvas.fill_color(50).
  rectangle(0, 0, page1.box.width, page1.box.height).
  fill

overlay_canvas = page1.canvas(type: :overlay)
overlay_canvas.fill_color(129, 192, 255).
  font('Helvetica', size: 50).
  save_graphics_state.
  translate(170, 330).rotate(30).
  text("Hello World", at: [0, 0]).
  restore_graphics_state.
  translate(170, 470).rotate(-30).
  text("Hello World", at: [0, 0])

form = doc.add(Type: :XObject, Subtype: :Form, BBox: HexaPDF::Type::Page.media_box(:A4))
form_canvas = form.canvas

form_canvas.
  fill_color(0, 102, 204).
  rectangle(0, 0, form.width, 50).
  rectangle(0, form.height - 50, form.width, 50).
  fill.
  fill_color(255).
  font('Helvetica', size: 30, style: :italic).
  text('Tutorial 2', at: [15, form.height - 35])

underlay_canvas.xobject(form, at: [0, 0])

front_page = doc.pages.insert(0)
front_page.canvas.
  font('Helvetica', variant: :bold, size: 50).
  text("Tutorial 2", at: [100, 600]).
  font_size(30).text("Modifying a PDF Document", at: [100, 550]).
  fill_color(0, 102, 204).
  rectangle(0, front_page.box.height - 50, front_page.box.width, 50).
  rectangle(0, 0, front_page.box.width, 50).
  fill

1.upto(10) do |number|
  canvas = doc.pages.add.canvas
  canvas.xobject(form, at: [0, 0])
  canvas.translate(70, 700).rotate(-55).
    fill_color(0, 128, 255).
    font('Helvetica', size: 90).
    text("Sample Page #{number}", at: [0, 0])
end

doc.write('modified.pdf')