why are F2803x I/O' s not 5V tolerant ?

I believe the process doesn't support the voltage level. Are you trying to connect as input or output? There are number of ways to implement this depending on your cost and performance requirements. I believe there are application notes addressing this question.

2803x high output voltage is minimum 2.4 V so it is ok with standart logic

but for inputs it says Inputs are not 5V tolerant

I am looking for an application note how to best interface 2803x inputs with 5V logic

A good report on this topic is titled  "3.3V DSP for Digital Motor Control" by Zhen Yu (SPRA550):  http://focus.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=spra550. 

Please check out the suggested application note. There are various level shifting devices which can be utilized as well. You can also implement discrete solution. It all depends on your performance and cost target.