A PSD cannot be viewed by anyone without Photoshop or a compatible editor. PDF looks the same on every device and keeps several pages in one file. PSD is at home in work-in-progress designs with layers; PDF is the usual choice for documents that must look the same everywhere, and printing.
Add several files and you can also combine them into a single PDF. The defaults suit most files. Under Options you can choose the page size and set the quality.
The PSD is converted as one flattened picture, with its layers merged; files with more than 8 bits per channel are not supported yet. Each picture becomes one page. The page can match the picture's own size or be A4 or US Letter; on paper the sheet turns to suit the picture, which is shrunk to fit inside the margins and never enlarged.