Package pdfpages: includepdf gives strange font on first included page. Only in Preview and Skim

Package pdfpages: includepdf gives strange font on first included page.
Only in Preview and Skim

I have a 4 page pdf abstract.pdf generated from another latex file that I
want to include in a latex document. I did it like this:
\documentclass[a4paper]{article}
\usepackage{pdfpages}
\begin{document}
\includepdf[pages=-]{../abstract/abstract.pdf}
\end{document}
My problem is that the text on the first included page looks strange,
while starting from the second page everything looks good.
Text from the first page of the original abstract.pdf:
Text from the first page of the included abstract.pdf:
Starting from page 2, the text looks fine again.
When I optimize the pdf filesize with the following GhostScript command,
the text from page 1 isn't even selectable anymore and only outlines seem
to remain:
gs -sDEVICE=pdfwrite -dCompatibilityLevel=1.4 -dNOPAUSE -dQUIET \
-dBATCH -sOutputFile=out.pdf in.pdf
I have this problem when opening the pdf with Preview and Skim. If I open
the pdf with Adobe Acrobat, the text looks normal and is selectable after
optimizing with GhostScript. What goes wrong? I would like my pdf to be of
equal quality when opening with Preview or Skim.
I am using MacTeX2013 on Mac OS X 10.8.4.