That Trump didn’t sign it doesn’t make it unenforceable. Trump could – but won’t – subsequently ratify it as being for his benefit. The Opening Arguments podcast, hosted by a real lawyer, explains how Trump is caught in a devilish catch-22. If Trump doesn’t affirm that he’s the pseudonymous person in the agreement, then Stormy Daniels is free to talk about her affair with Trump. If Trump does affirm that he’s the person in the agreement, then of course he’s admitting that he did engage in a settlement with her, laying waste to the repeated protestations that this never happened, this is fake news, etc.
The Opening Arguments host also points out that this isn’t just a nondisclosure agreement, it’s a settlement agreement.
And yes, moral questions aside, there’s a legal problem because paying Stormy Daniels $130,000 to keep quiet right before an election is arguably a benefit to the Trump campaign that went undisclosed.