In rare cases, a Mac or PC's operating system can be configured so that the file type association for a PDF (Portable Document Format) file is not set properly. PDF is a very common and popular format for print-ready documents that are distributed within a software application (such as SimUText), or on a web site.
In most circumstances, your computer will be set up to launch the Adobe Reader application (or the Preview feature, on a Mac) when you try to open a PDF file.
If you have trouble generally opening PDF files, use your computer's Help system to investigate whether the PDF file type is associated with something besides the Adobe Reader application (or Preview), or does not have any application associated with it. If so, follow the instructions that come with your operating system to set the association for files with the ".pdf" extension to open in the Reader application. Installing the free Adobe Reader application should also set the association for you.