You are right in thinking that the font size of the CHM output is controlled by the style sheet. Also CHMs can not be used in conjunction with the Ctrl + Mouse wheel to increase/ decrease the zoom level. This is available with web based output (e.g Webhelp). So as far as your options are concerned, it looks like changing the font size in your .CSS or deploying webhelp and allowing the users to set their own zoom level.
Read the RoboColum(n) for mutterings on RoboHelp, Technical Communication Suite and technical communication.
Perhaps create another CHM layout that uses a .css with larger fonts, and provide that CHM together with the standard one?
Then provide a link or button in the standard CHM (in the toolbar or in each topic?) labeled "Large Print Version" or some such?
A bit kludgy, I know, but an out-of-the-box solution might be your only option.
Thank you for your response. Much appreciated.
Can you tell me if I can still add the font button. I have a .chm that I found on the web and it works fine with the font button. I just can't get it to work for me.
Take a look at Peter Grainge's Reverse Engineering topic, for several ways of decompiling that .chm file. You'd then have better luck analysing how they accomplished that.
Leon's advice is always good, but in this case it unfortunately won't help you achieve the goal.
This is because some "magic" is involved in making a Font button appear. It may not even be possible to achieve any longer with RoboHelp. I haven't tested it in Eons. I've documented the steps involved in my Tips and Tricks file available at RoboWizard.com. Click here to visit the site.
Thank you for all your replies. Even with the tips and trix, I couldn't get this to work with RoboHelp. Seems to me it's a great little feature though and this wasn't the first time one of our customers had requested the ability to change the font size. Can Adobe not put it on a list of change requests for the next release? (I've no idea how one gets requests to them) You wouldn't think it would be too hard to code.