If you copy-and-paste text from another program (Word and the like), use only unformatted text otherwise the formatting wil be copied also.
This is text from Word with style ‘Normal’.
This looks like ordinary text, but the fontsize and margins are hardcoded.
This is text from Word with style ‘Heading 2’.
And this contains even more formatting.
So don't copy formatted text. You can get rid of the formatting by pasting the text in Notepad and copy the text from there.