The default output encoding used by LaTeX is OT1 and fonts in this encoding don't have precomposed accented characters. Therefore
ä is first transformed into
\"a which becomes
\accent "7F a. Thus in the PDF file we get a set of elementary instructions for superimposing the dieresis to the
so the fonts used will not suffer from this problem, because they cover the glyphs used in several European languages written with the Latin alphabet. Swedish is completely covered.