Difficulties opening PDFs in Preview
Preview has 2 settings for opening files.

One is Open all files in same window

the Second one is Open all files in separate window.

The open all files in one window side bar will show all files, and will add a new open file toit.

But, check your settings for Sort by in the side bar, pending those setting the new file might not open at the bottom of the list, so with 15 files open you might miss it.