If all the whitespace is regular space characters this should be simple with Find & Replace. Open the Find & Replace window from the Edit menu or by pressing Ctrl+F. In the "Search for" field enter two spaces followed by a plus sign, meaning "two or more spaces":
_ _ + (underscore used here to represent a space)
In the "Replace with" field enter a single space. Press the "More Options" button to show additional options at the bottom of the window. Check the option "Regular expresions". Press the button "Replace all".
That should take care of all the whitespace. If it doesn't you may have unusual whitespace characters. You may be able to copy/paste such characters into the "Search for" field to remove them by the same method. (Leave the "Replace with" box blank if you just want to remove characters and not replace them with a space or anything else.)
To replace line breaks with a space, enter
\n in the "Search for" field, a single space in the "Replace with" field, again make sure "Regular expressions" is checked, and hit the "Replace all" button.