I was refused a French visa and now I got a refusal from the Latvian Embassy. I do not understand the reason:
The information submitted regarding justification for the purpose and conditions of the intended stay was not reliable.
When I apply to the Embassy of Portugal, how do I explain? Is the previous refusal letter necessary in a new file or not?
You already look like you are just trying every Schengen country until you find one that will give you a visa, a process sometimes called 'visa shopping'.
All those countries will know about each others refusals and so you will need an extremely good reason to go to Portugal. The meaning of the refusal is they don't think you're visiting for the reason you gave, and you're likely to overstay or perhaps commit crime.
If you are a business traveler, you will need to consider hiring a lawyer to help with your next application. The expense may be justifiable.
If you are a tourist you should consider building up a travel history away from Europe first as this can improve your credibility in future applications.
