PDF is the standard format for emailing portfolios, but make sure that the file size remains relatively small - 2MB or less.
Large files may clog up a practice's email system or simply fail to be delivered.
Most practices prefer digital format portfolios for initial job enquiries or applications, but will usually want to see a paper portfolio as well if you are invited for an interview. Check which format the practice prefers if you are unsure.
Keeping an up-to-date virtual version of your portfolio ready will allow you to respond quickly to job adverts and is far cheaper than printing out a portfolio for every application.
Many job adverts receive hundreds of applicants, so sending your application in early will help you reach a recruiter before they are overwhelmed with applications.