A text file is double-spaced if there is a blank line between each line of text. Select Single -> Double Spacing from the Convert menu to insert a blank line between each line of text, making the current file double spaced. If you select multiple lines of text first, only the the selected block is converted. If there is no selection or the selection does not span multiple lines, then this conversion affects the entire file.
Note that EditPad Pro will not force you to maintain the double spaced format of the file. Pressing Enter will insert a single line break as usual. Press Enter twice to insert a double line break. You can type text into the blank lines added by the conversion if you want. This could be useful to annotate a manuscript, for example.
Convert|Double -> Single Spacing