The Public Domain contains works on which copyright has expired. Basically you can do nearly anything with it. Does Public Domain therefore count as Open Source or free/libre?
Public domain is indeed free/libre. This includes CC0 and the Unlicense, which are commonly used as public domain dedications.
Being in the public domain is not a license; rather, it means the material is not copyrighted and no license is needed. Practically speaking, though, if a work is in the public domain, it might as well have an all-permissive non-copyleft free software license. Public domain material is compatible with the GNU GPL.
If you want to release your work to the public domain, we encourage you to use formal tools to do so. We ask people who make small contributions to GNU to sign a disclaimer form; that's one solution. If you're working on a project that doesn't have formal contribution policies like that, CC0 is a good tool that anyone can use. It formally dedicates your work to the public domain, and provides a fallback license for cases where that is not legally possible.
As for open source, refer to OSI's FAQ. Note that the OSI did not approve CC0 because of the explicit lack of patent grant.
TL;DR: public domain is both free/libre and open source.
There are multiple types of public domain. All can be viewed as open source (when there's source), but not all of them are free.
The first type is works whose copyright had expired. Note that different countries have different copyright law, so anything newer than XIX century may happen to not be in public domain in one or more countries. Works that are not copyrighted anywhere anymore are free in the same sense as works under GPL or MIT or another free license. If it's still restricted in any country, it's not free in the same sense.
The second type is works that were declared public domain by their authors. The problem with it is that in most countries you cannot declare your own work public domain, and the definition of public domain is restricted to particular cases like works created 70 years before the author's death or works by the government, so this claim is technically legally void. Even though the author has no intention to sue anyone over it, there is no clear agreement between you and the author regarding what you can do and what you can't either, which may be important in some cases.
The third type, which is not really public domain, is works released under CC0 and similar permissive licenses. These are indeed free.
Also, while it's hypothetical at this point, if some closed-source binaries survive until they meet the definition (1), they will be public domain but still closed source, there will not be any legal obligation for author's descendants to open source it.