First the usual warning. You can do this in Acrobat (if you don't have the original files), but it's not the best way to do this. These changes are always best made to the originals.
OK, try using the TouchUp Object tool. You can select an object (say, a text panel), copy it and paste it into another document. You may need to use the TouchUp Text tool to clean up any extra stuff that gets copied over.
Also, you could insert text into a transparent text box with no outline (just like Word).