Should the pdf viewer be opened in the background.
Value
This function is executed for its side effects. The specified PDF file
is opened in the PDF viewer and TRUE is returned.
Details
Currently this function works on Windows and Unix platforms. Under
Windows, whatever program is associated with the file extension will
be used. Under Unix, the function will use the program named in the
option "pdfviewer" (see help(options) for information on
how this is set.)