Generally speaking, converting all color to RGB when creating a PDF file isn't going to hack it either in terms of how transparency is interpreted.
Also understand that Apple's PDF previewer doesn't implement anywhere near the full PDF specification and is especially problematic with color and transparency.
The free alternative is to use the Adobe Reader on both iOS and Android devices. CMYK is reasonably displayed with Adobe Reader (no garish colors and overly high contrast from CMYK or grayscale) and if you print from those devices, you don't have problems of messed up CMYK values, rich black, etc.
- Dov