I have an XML that looks like this:
<Description><![CDATA[Adds or subtracts some interval of time from a date or time.]]></Description>
<Syntax><![CDATA[DateAdd(interval, number, date)]]></Syntax>
<Notes><![CDATA[Interval may be:
y=day of year
ww=week of year
Since you want to remove all indentation, you don't need an XML parser/writer.
You can load the text as separate lines using File.ReadAllLines, and then process each one of them with string.TrimStart. Then save them back to disk using File.WriteAllLines.
N.B.: File.WriteAllLines appends a newline at the end of the file (since it follows each line by a newline); if you don't want one, you should join the lines yourself.