Text files are just a convention: they have no "real" concept of lines at all - that is provided by the application which processes them, by looking for what it considers as "line end characters". And these vary from system to system (and can even bee application specific in some cases). In all other matters, they are binary data which uses a limited set of the available values.
So the concept of line spacing is not something a text file can work with: if it doesn't know what a "line" is, it can't space it!
If you are trying to read a .TXT file into you application and display it then exactly how you change the spacing depends on the control you are using to display the data. For a multiline textBox, I don't think it is possible, for a RichTextBox it's pretty simple:
Changing the line spacing in a RichTextBox control[
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