It can be useful to cut and paste via a plain text editor to get rid of any unseen control characters. I.e. Cut from the original, paste into the text editor and check for unusual stuff, then cut from that and paste into Scribus (or whatever else).
It's a little bit more work - a couple of extra mouse clicks or key presses - but at least you can be pretty sure that you're not copying over anything that could cause weird problems.
You never know what's being copied to the clipboard. Try copying some formatted text from, say, Firefox - for instance, this page - into, say, OpenOffice and you'll probably see that some formatting has been included. Applications try to be helpful but sometimes you just want them to be dumb.
P.S. The Firefox add-on "Copy Plain Text 2" is brilliant for this kind of thing. A real time saver. (I'm not sure how well it works copying from a PDF displayed in Firefox though. That'll need some experimenting.)