Conditional Logic can be set in the Template Editor when creating a field. At the bottom of the field edit box is the conditions box. Only text, integer, boolean, select, or agreement field can be used with conditional logic. Select which question and answer will trigger the field to display by choosing from the drop down selection. Conditional Logic only works within each section and not across various sections. Click Save when done.
If you do not want the labels to show for your conditional fields in the web portal or in mobile, go to Admin>Settings and uncheck the box for "Show labels for conditional fields". These fields will not show in the PDF Reports either if you have this box unchecked.