The key requirement here would be that the user has a PDF writer on their PC. Then if they do, all they need to do to print a specific topic is click Print in their browser, the PDF writer will appear in their list of printers.
Before allowing them to print the entire help with one click, you need to consider its size. I've seen projects with thousands of topics and sending that to a printer might not make you the most liked person in the office.
The usual compromise is for you to create PDFs of sections of the help and create links to them, either text or icon.
To create a PDF of the help in whole or part, use Printed Documentation. In theory you can generate direct to PDF but I recommend outputting to Word first and tidying up the page breaks and so on. Then create the PDF from Word. If you haven't done this before, see Printed Documentation on my site.
See www.grainge.org for RoboHelp and Authoring information

@petergrainge
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