I have some RTF files, created by various means, some in DTP, some outside. Most seem to have a fixed setting for the right margin meaning that the text does not expand to fit the area available in the window. I just pasted some text into one of these RTF files with fixed width though and for this text the downward arrow that marks the right margin moves as I expand the window, so that the text fills the available space.
I would like all my RTF files to work in this way, i.e. text expands to fill the available space, but can’t work out how to adjust the files I have to behave in this way.
The effect you want is called wrap to window. It is the default in new DTP RTF files. The converse of wrap to window is wrap to page. If you select a block of text and move the margin point (the downward pointing arrow) anywhere to the left of the width of the ruler, then wrap to page is set for that block of text. To set the block of text to wrap to window, select it and drag the margin point as far to the right as you can. It doesn’t matter how wide the window is when you do this, just drag the margin point all the way to the right.
(Wrap to page / wrap to window is an explicit format menu command in TextEdit, but not in DTP.)