If you can't trust a news source that makes retractions, then there are no news sources left to choose from. The Daily Mail, which has a long list of retractions, can't be trusted by your own logic. So if the Daily Mail claims to have a signed document showing anything, how can that be trusted.
The fact that a news source retracts a story on its own is not a reason that the source cannot be trusted. In fact, retracting something that they got wrong is evidence that source isn't simply lying, otherwise they would claim that whatever source shows they are lying is the one that is in fact lying. That's the whole basis of the idea of fake news.
I've done some research on news sources and their reliability rating. Newsweek and CNN are both considered reliable sources while the Daily Mail is banned as a reliable source (you can find reliability ratings on news sources with a search of the internet, I won't do that for you). Their claim to have a document stating anything is questionable because of that. Even if they are in possession of that document I don't see any proof in the article that the Daily Mail is in possession of anything but a piece of paper. Additionally, the claim is that they have a "statement" in which she says that the affair did not happen and that she isn't just saying that because she was paid hush money. A statement is not a legal document, a person making a statement is free to lie, whether it is about the affair or the hush money, without legal consequence. However, the statement contradicts statements she has made previously, so how are we to determine when she is telling the truth. It also doesn't make any mention of why she was paid $130,000, other than that it wasn't hush money (If I was paid that kind of money and was asked to make a statement saying that it wasn't hush money then I would do it, no real repercussions).
Its my opinion, but the motivation for releasing this statement to the Daily Mail, which is not a reliable news source, instead of any reliable news source, is that the statement can be easily brushed away later if evidence to the contrary becomes available. Or from another perspective, only an unreliable news source is willing to publish the statement as that source has already shown that it is less interested in the veracity of the content than it is with the number banner ads that get viewed on their website. If the content of the statement can be verified then I would expect to start seeing being reported by many other sources where her statement itself is true or not.
I don't care who Donald Trump has sex with. I do care about reliable news sources being discarded in preference of unreliable news sources.
I didn't say anything about Fox News. My opinion is that they have a heavy bias, and I don't find them to be as reliable as other sources, but they are certainly more reliable than the Daily Mail.