I have just checked -- the code below worked perfectly fine (Win7 64-bit, VFP9, Adobe Reader X)

DECLARE INTEGER ShellExecute IN shell32;
	INTEGER hWindow, STRING lpOperation,;
	STRING lpFile, STRING lpParameters,; 
	STRING lpDirectory, INTEGER nShowCmd

ShellExecute(0, "open",;
			"C:\...\myfile.pdf",;
			"", "", 1)