If you highlight and copy text from a document using the document viewer and then paste it into another application, the formatting may alter. It may also contain different characters than the original selection. This often happens when copying text from a PDF document with multiple columns.

This problem happens because of how some document formats handle text. The actual text in the document is stored differently from the way it is displayed. This may result in a copy that does not appear as expected.

Unfortunately, there is no real way of fixing this problem. Copying less text at a time, or copying the text into a text editor (like gedit) may minimize the problem.