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.