PDF is the file format, we'd very much prefer to receive.
The reason for this, is that when you save a PDF-file on a computer, a package of all the components and composites, that are included in the design process, is made.
This means that all the information regarding colors, fonts, font sizes, line height, and all the other details, is saved in the PDF - and when you send it to us, we also have that information!
This also explains the name of the PDF: Portable Document Format.
Fonts and colors has to be embedded in the PDF. This way, it'll cause less problems down the line, since we wont have to track down something which wasn't received initially.