"The doctor's letter is always hard to understand."
Translation:A letra de médico é sempre difícil de entender.
This doesn´t sound natural in English. We would say ´the doctor´s writing is hard to understand´.
According to my dictionary it should be "carta", not "letra". "Carta" is for correspondence, "letra" is for letters of the alphabet - A, B, C etc. ........ see reply below. Once again the 'hints' don't tell the whole story.
The English sentence probably came from a poor translation of the Portuguese and should have been more like "The doctor's handwriting is always difficult to understand".
In fact, the dictionary translates "letra de médico" as "scrawl". :-)