How do I place a link to other pdf documents within Docbook

Hi again,

 

I have reviewed the docs online and would like to know
how to place a link inside a DocBook document which links
it to another PDF document. Which tag do I use and where should I save the other
referenced PDFs?

 

For example in DocBook:

 

“Please refer to User Manual.pdf” where the User Manual is a separate
document

 

Thanks,

 

Sue