Theres lots of alternatives, what exactly are you trying to do? use a selected combobox to open a pdf?
personally i would be tempted to take the route of the openFileDialog and allow the user to select the file they want to open by navigating through their files, something you could limit to being only pdf's using something similar to
- openFileDialog1.Filter = "txt files (*.txt)|*.txt";
where the txt would be your pdf tag instead
its what users would expect to come across when wanting to open a file and with windows forms keeping to whats expected is always good for your users :P
i didn't go too indepth in code here because presumeably you already know how to open a pdf using file.io as that wasn't part of your question!