Most users looking for pkpass to pdf conversion usually just want a printable version of their Apple Wallet / Passbook tickets (.pkpass files). Unfortunately, no such program just for printing pkpass to PDF exists and some most likely goes for some kind of passbook converter.
However, services based on Apple Wallet / Passbook usually provide users with PDF files in confirmation e-mails, as well as the possibility to print additional PDF ticket from the users account. You can open your own .pkpass files in the default app on iPhone (Apple Wallet), unfortunately there is no official alternative for this on other platforms or devices, so you will have to look for 3rd party solutions.
Last but not least, a .pkpass file is just renamed ZIP file that you can actually decompress in Windows (rename the suffix so it is recognized). If you do that, you can usually find a .png with the ticket inside and you can print that to PDF easily.Open PKPASS file Open PDF file
A digital pass managing utility integrated in iOS 6 and above
A digital pass managing app for Mac