user6726's answer correctly notes that probable cause is not required to issue a grand jury subpoena. However, this is not because the bar is lower for a grand jury. It is because probable cause is the standard required by the Fourth Amendment to issue a warrant, and the Fourth Amendment does not apply to subpoenas.
The nature of a subpoena
Rather than a warrant to arrest or search the property of a suspect, a subpoena is an order to a witness to produce documents or provide testimony. Subpoenas are available in civil as well as criminal proceedings, and can be obtained by any party. Normally a party who requires the disclosure of documents under subpoena before trial must show that (1) the documents are evidentiary and relevant, (2) they cannot otherwise be procured with due diligence, (3) they are needed for trial preparation, and (4) the application is made in good faith and is not a 'fishing expedition': United States v. Nixon, 418 U.S. 683 (1974), pp 699–700. As pointed out by user6726, a lower standard applies before a grand jury.
In criminal investigations, most subpoenas are aimed at third-party recordholders: Slobogin, 'Subpoenas and Privacy' 54 DePaul Law Review 805 (2005), p 809. A third-party subpoena eliminates the possibility the suspect will destroy the evidence and normally eliminates the target's ability even to challenge the government's action: Slobogin, p 811. A typical example is a subpoena directed to a bank to produce a suspect's financial records. We don't have the facts yet, but in this case, it seems likely that a subpoena has been issued to the Trump Organization in its capacity as a third-party recordholder.
Subpoenas to witnesses who are also suspects
I originally stated that a subpoena is directed at a witness rather than a suspect. As pointed out in the comments below, a suspect can be a witness as well. But at common law, a subpoena could not be issued to a defendant in a criminal case because of the privilege against self-incrimination: R v Purnell (1748) 1 Wils 239, p 243. In Boyd v. United States, 116 U.S. 616 (1886), pp 634–635, the Supreme Court held that the Fourth Amendment protection from unreasonable searches and seizures and Fifth Amendment privilege against self-incrimination reflected this part of the common law.
In a line of cases summarised in Slobogin, pp 814–821, the Supreme Court departed from Boyd and limited the scope of the Fourth and Fifth Amendments. Significantly, in Hale v. Henkel, 201 U.S. 43 (1906), p 73, the Court held that the Fourth Amendment does not apply to subpoenas, and the Fifth Amendment does not apply if the witness is immune from prosecution. Later, in Fisher v. United States, 425 U.S. 391 (1975), p 408–410, the Court held that the Fifth Amendment does not apply to a subpoena to produce self-incriminating documents unless the act of production has a testimonial aspect.
In this answer, I originally stated that the Fifth Amendment would generally make it pointless to issue a subpoena to a suspect. That took the point too far, but United States v. Hubbell, 530 U.S. 27 (2000) demonstrates that the Fifth Amendment still imposes significant constraints on the use of subpoenas against suspects by requiring prosecutors to grant immunity to witnesses in many cases. A more accurate summary appears in Cole, 'The Fifth Amendment and Compelled Production of Personal Documents After United States v. Hubbell – New Protection for Private Papers?' (2002) 29(2) American Journal of Criminal Law 123:
[T]he net result of the Court's decision in Hubbell is to limit the practical ability of prosecutors both to compel production of documents and to use the contents of those documents in prosecuting the witness who produced them. As a result, prosecutors conducting criminal investigations may be more likely to use search warrants, rather than subpoenas, to obtain documents from individuals.