As FumbleFingers noted in their comment, the usual (certainly in British English, and I think in American English as well) form is pronounced dead, though declared would probably be understood. Common things that one might come across (some current, some historical) include:
X was pronounced dead at the scene (of the accident/crime/etc.)
X was pronounced dead on arrival at <name of hospital>
The surgery was not successful, and X was pronounced dead at 17:35 (or whatever time)
When someone is pronounced dead, it means that medical professionals have evaluated them and determined that they meet the generally understood meaning of dead, and that there is no further purpose in resuscitating them. It is an expression of the professional judgement of those involved.
To pronounce, generally, has two main groups of meanings. One related to speaking aloud, and refers to the manner in which one speaks. The other relates to making official statements of various sorts, which might be declaring the legal result of some procedure, as in "I now pronounce you man and wife", doctors making a professional judgement with legal importance, such as a death, or a judge handing down a judgement of some sort, as in "pronouncing sentence" when sentencing someone who has been found guilty. Declare is also used for this sense of official pronouncement, so people will often readily consider pronounce and declare interchangeable in this sense. Their other senses are also related, but are not so interchangeable.
On the other hand, someone's death might be announced, meaning to let it be known publicly that the person had died (and perhaps some details of how they died). That would not generally be the job of doctors, though. It would usually be done by someone's family, employer or agent.