I thought that this was an obvious problem and that it would be fixed in the next DTTG version. However, since the problem still exists for me, I am not sure if this is a general problem.
I am using the latest iOS and as soon as I open a PDF document in DTTG it displays the PDF inverted. That is, black background and white font (other colors are also inverted). If I open the same PDF from DTTG in another program (eg PDF Expert) it is displayed correctly.