After downloading the form, MS Word may open the file in Viewing mode. To enter Editing mode, click on View in the top menu and then choose Edit Document in the drop-down window as shown on the left.
Depending on your system, it may appear differently. For example, you may see it as a yellow bar with a button in the top middle of your screen:
Your security and privacy are important to us; the file is absolutely safe to be downloaded and opened.