Which format is the best for print?

[Big Frank]

I've designed a postal card but when I printed the file, the quality was darker than the original one in screen

What did you print this card on and what did you print it with?

[Albacore]

My question is very similar to Frank's. Are you using your own printer, desktop printer, or are you sending it away to a print shop to be printed? If it is the later then go with #post 2 suggestions along with #post 3. Also if your are using a certain print shop frequently get a colour chart printed but make sure that each colour has also it values printed as well so that you only use a restricted palette and that you know when you pick a colour what it will look like when printed in CMYK. If it is the first then through your printer driver control the flow of ink when your printing on thick card. Also either here or on the internet do a search on printing in CMYK from RGB, also look at your PDF export in Acrobat. Even from true CMYK programmes from Adobe certain colours will always give you some cause for concern.

[printbucket]

You may want to familiarize yourself with different paper stocks. Different paper stocks deliver different result.

[printchesco]

Different stocks print differently because of the ink absorption so you can't expect prints to turn out identically for different papers. That's why it's good to have some paper or cards in stock for making test prints, so that you can make adjustments to your designs before you print the final output.