Use the shellexecute function:

Declare INTEGER ShellExecute ;
IN SHELL32.DLL ;
INTEGER nWinHandle,;
STRING cOperation,;
STRING cFileName,;
STRING cParameters,;
STRING cDirectory,;
INTEGER nShowWindow
nWinHandle=0
cOperation="Open"
cFileName="my.pdf"
cParameters=""
cDirectory="c:\pdffiles\"
nShowWindow=0
=Shellexecute(nWinHandle,cOperation,cFileName,cParameters,cDirectory,nShowWindow)
or
=Shellexecute(0,"Open","my.pdf","c:\pdffiles\","",0)