Ford pardoned Nixon without any legal charges made against him.
BHO cannot pardon HRC because she was never charged, let alone convicted, of any crime. Secondly, were he to attempt to do so, it would be a de facto admission of guilt.
https://www.law.cornell.edu/supremecourt/text/71/333#writing-USSC_CR_0071_0333_ZO
...The Constitution provides that the President “shall have power to grant reprieves and pardons for offences against the United States, except in cases of impeachment.”
The power thus conferred is unlimited, with the exception stated.
It extends to every [Federal] offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken or during their pendency or after conviction and judgment.
This power of the President is not subject to legislative control. Congress can neither limit the effect of his pardon nor exclude from its exercise any class of offenders...
From Wikipedia: “A presidential pardon may be granted at any time, however, and as when Ford pardoned Nixon, the pardoned person need not yet have been convicted or even formally charged with a crime.”
Ford pardoned Nixon without any legal charges made against him.
Ford pardoned Nixon “in advance.” Legally dubious but it has not been tested or challenged. If Obama did it I expect it is not worth challenging either. But, a pardon won’t necessarily stop an investigation that tries to uncover the truth. The investigation should proceed regardless.