On the Form tab in the dialog box of the ribbon command File | Properties 
, you can enable or disable the form mode and secure it with a password if desired.
If form mode is enabled, you can only fill in form objects (text fields, checkboxes, dropdown lists, etc.). The rest of the document cannot be changed.
Note: When you save a document in form mode, it will be opened again in form mode.
You can enable or disable form mode via the option Form mode active.
Tip: You can also use the ribbon command View | Form mode 
for this purpose.
If you enable form mode and also enter a Password to deactivate, anyone who subsequently attempts to disable this mode will be prompted for the password. Form mode can only be exited if the user enters the correct password.
Important: If you save a document with a password in form mode and then forget the password, you will no longer be able to disable this mode. Thus, note the password and keep it in a safe place!
For more information on working with forms, see Forms.
Shade form objects
This tab also has an option called Shade form objects. If it is enabled, all form objects (text fields, checkboxes, dropdown lists, etc.) are highlighted in the text with a light gray shading. This makes them stand out better from the rest of the form.
Tip: You can also use the ribbon command View | Shade form objects 
for this purpose.