In order to save your MDL files to PDF, you will need to install virtual printer software. For this job I can recommend CutePDF Writer as it's the most comfortable option regarding PDF printing and exporting. When I mention printing, I refer to virtual printing.
Simply download the printer driver and install it then use this application to open MDL files and use the print function and choose CutePDF Writer as the printer. Click OK, enter the name of the file and your MDL will be converted to PDF file format.