Adding an index to your pdf-file

Have you ever created a pdf-file from multiple documents? And wanted to have a clickable index for the pdf-file?
on more than one occasion have I created documents that I need to make an index to, and after some gogling I have come up with the solution that I will share in this article.
We only ned a couple of files and programs to make this work.

First of all we need the original pdf- file we want to add an index to.
Secondly we need to know what index title goes to which page.
Third and most important is the gs command. if it's missing we'll install it using:

sudo apt-get install ghostscript

We then create our index-file (index.info):

[/Page 1 /View [/XYZ null null null] /Title (First page) /OUT pdfmark 
[/Page 2 /View [/XYZ null null null] /Title (Table of Content) /OUT pdfmark 
[/Page 3 /View [/XYZ null null null] /Title (Document content) /OUT pdfmark 
[/Page 6 /View [/XYZ null null null] /Title (Appendix A) /OUT pdfmark 
[/Page 8 /View [/XYZ null null null] /Title (Preface) /OUT pdfmark 

This file contains the index for our pdf-file. We can add more pages and titles as we like simply by changing the page number and title within the (). To create our new indexed pdf-file we simply run the following command:

gs -sDEVICE=pdfwrite -q -dBATCH -dNOPAUSE \
-sOutputFile=newfile_indexed.pdf index.info \
-f original_unindexed_file.pdf

We now have a new pdf-file with a clickable index.