Presumably you are asking about saving multiple images in a contact sheet layout to file in LR's Print module. From this module you can only write jpg files and you will have to use another program to convert the jpg to pdf.
You can also print to any printer installed on your system so if you have Acrobat installed you can print the contact sheet to PDF or you can search the internet for "PDF Printer" and there are many free PDF Print drivers out there you can test to see which suits you better.
If you are using Windows you can also use the Microsoft XPS Document Writer and the free XPS Document Viewer is available for download from Microsoft.
Thanks! Most helpful