Is there a possible way to open a file in visual basic by just putting the name of the drive?

Recently I made an application that has lots of PDF files in it and I made a setup for it using Inno Setup Compiler. In the setup, I allowed people to change where they want to install the app. For opening my PDF files, I used: system.diagnostics.process.start("My pdf.pdf")

My problem is that in the code above, I put drive "C:" and when my user changed the install directory to drive "D:" the pdf's did not work and the error showed that "Cannot find the specific file". My question is that is there a way to just put the name of "computer" or "a drive" in the code above, not the specific name of the pdf, and let the computer find the file itself?