In Acrobat, one can use the link tool, but this might not work in a form completely built in LC Designer.
i´ve found an solution but it is not very nice, because at the moment it only works in an browser.
The link in my pdf is only a http-send button without any boders or color, so that you can only see the text. This button points on the pdf that you want to open (insert path in the url-field e.g. ../path_to_pdf/sample.pdf)
This not the best way to do this nut it´s the only solution that i´ve found. I hope this helps you.
There are a lot of 'how do I insert a link' postings on this forum. Have a look at this one for instance:
Using the sample referred to by Steve, I found it worked better by transferring the script from the button to the drop-down list in the mouseExit event. To be more user friendly, change the Drop-down list Values to "Adobe", "Google" etc, rather than the web addresses themselves. For these go to the bindings tab, select Specify Item Value and insert the web addresses into the corresponding Values.