I have designed portfolio at 72 dpi, so when I exported as pdf got horrible quality. Can I do anything to get normal quality? I exported png which is fine when displayed in browser but in picture viewer you have to zoom (because it's too long)
Unless everything you do is for screen rendering (web/Mobile) then you should design at a higher resolution, and down-save to lower resolutions for output. Even some mobile devices require designing at more than 72ppi.
There's really no fantastic method for making a low resolution image appear good at a higher resolution. You can always reduce quality, but it's nearly impossible to improve quality in any sort of satisfactory manner.
No, not really.
If you designed it at 72dpi you CAN increase it to 120-300dpi but then you will get horrible pixelation and artifacts.
You should always design LARGER than you intend and scale down if needed. That way you won't lose as much quality during the process.