Where is the PDF when you check the box when saving.

When saving an Adobe illustrator filed format, they have the check box to create a PDF file. This makes the file larger. So where is this PDF file stored? Is it a standalone? How does somebody view the PDF that is created?

I had an issue with a file that I didn't save it and I would get a pop-up message that it couldn't be loaded because the PDF wasn't created.

I have a very large file for a backdrop and the company said I could either send it as a PDF or  illustrator file.  

I've had a heck of a time trying to get the file finished just because it would seem to crash many times, but now I finally have it and trying to decide if I should be flattening the image because there's a drop shadow. I'm assuming a PDF file is a flattened image? That's why I wanted to see what this file was.

I have bleed outside of the artboard and I wanted to know if it's automatically trimmed. I don't want to confuse the printing house if say the guides don't show and if the bleed becomes the new size. They'll think that I want it that way, but I left the bleed because of the hemming. Normally the company has a way of cutting and pasting by flipping an image over for their hemming. But occasionally it makes mistakes. So I figured since I had extra image I would let it overhang the artboard.

So I didn't know how a PDF handles this. And like I said, I don't know where the file that is created as a PDF when you check the box lives. Thanks for letting me know.

By the way, as far as flattening, one representative, there said two flat in the image. One said it didn't matter. They would let their design team look at the file.