If anybody is interested, the MATLAB open doesn't appear to support PDF files in Linux. I ended up using the CentOS Linux command "evince". Other flavors of Linux may have another command to open PDF files.
if isunix
system(['evince ' help_file_name ' &']);
elseif ispc
open(help_file_name);
else
msgbox(['Open ' help_file_name 'in the operating system'],'Product Help');
end