PDF Invalid Characters

In case the PDF file is showing some strange character, you need to use a different font for the PDF. The default font used is Helvetica, a light font but that doesn't support all the languages. Inside the full package archive of VikBooking that you downloaded there's a folder called Dejavusans_PDF_Font. Upload all the files contained in that folder (not the folder, just the files inside) onto the following directory of your server:
/components/com_vikbooking/helpers/tcpdf/fonts/
This way VikBooking will use the font Dejavusans and every character will be encoded correctly. The reason why the program doesn't come with this font pre-installed is because it weighs about 5MB, too much for a Joomla installer.