My print file is exported in a larger size than the original when I want to save it as a PDF, JPG or

Hello,

I've set up a file for printing at 300x300 millimeters, 300 dpi and when I want to export it as a PDF, the file gets bigger and measures 850x850 millimeters, which is much too big and doesn't correspond to the original size.

I also have exactly the same problem with Photoshop and Indesign.

I don't understand why my file is getting bigger. Would you have a solution for me?

Thanks in advance.

Julie