No, they are not necessarily considered corporations.
LLC actually stands for Limited Liability Company --- not Limited Liability Corporation. An LLC can be either a single member company which is disregarded as a separate tax entity, or it can be a partnership, or it can be a corporation. That is why you have to have them fill out the W-9 form to clarify their status.
On the W-9 form, if they check that their company is an LLC, then right next to that line they are further asked to clarify if they are either
A) A disregarded tax entity
B) A corporation
C) A partnership
If they check any box other than b, then you send them a 1099 form.
The designation of PC stands for Professional Corporation, and generally firms that have that designation as part of their name, are in fact registered as corporations. But again, the requirement is that you have them fill out the W-9 to have them actually indicate their tax status. Someone could be using the designation of PC when it really does not apply to them.
A W-9 must be filled out by anyone who provides services to your company. Anyone who does not check the box to indicate they are anything but a corporation must be sent a 1099 form if you paid them more than $600 for the year.