JOB Assuming you are on windows you can use the following code snippet to open e.g. a PDF file with it's associated application.
set pdf_file "C:/test.pdf"
# the following code -does not- work on all PC's (?) ...
# set command [list {*}[auto_execok start] {}]
# exec {*}$command acrobat $this(exported_PDF_file) &
# -works so far-
# open a file (.xls, .pdf, ...) with its usual application:
set command [list {*}[auto_execok start] {}]
exec {*}$command $pdf_file &