DITA is a document architecture, not a tool. Thus, this does not limit your choice for a development tool or for creating PDFs and online help from your DITA source. It's unclear what kind of on-line help you have in mind, but it looks like you want HTML pages that can be displayed by the application itself than trypanes like CHM.
There are several tools that will generate your exits from DITA. DITA-OT (Open Toolkit) is a Sourceforge FOSS tool that can create XSL-FO, from which you can create PDFs, as well as HTML, CHM and a few more. Then there is the free DITA2Go http://dita2go.com , which makes Word RTF for PDF production, as well as HTML and several forms of online help; it uses the theme identifier attribute as part (or all, if you want) of the HTML file name. In addition, there are commercial programs, although not with a big advantage over DITA2Go.
Itβs best not to think that PDF is an on-line delivery format, but just like a version that customers can print and use as a hard copy. And tooltips really suit your web application more than a help creation tool.
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