Some printing machines can print colors on top of others and some cannot. Some PDF viewers offer an optional preview function that simulates the effect of overprinting on a display screen. Since this function is not specified and therefore implemented differently or not at all, it causes some confusion among users.
In some writing systems you can place the characters vertically from the top to the bottom to form you sentences. Most users know this from Chinese, Japanese and Korean texts. What does it take to use the vertical writing mode in the PDF? And, what does it mean for a viewer to display the characters of vertical text correctly?
The native font format of Microsoft Windows is TrueType. The PostScript fonts are not very well supported on Windows and the Type 1 format has even been discontinued. On the other side some embedded computers of printing machines have troubles with TrueType fonts and prefer PostScript fonts. One approach to circumvent these problems is to convert the font to the desired format but this can be a challenge of its own.