How can I view contacts I backed up with Xperia companion on my computer?

It is not possible to view an existing backup file done by Xperia Companion directly on your computer. However, contacts which were backed up from your Xperia device can be transferred to your Google account and viewed from there.

To transfer contacts from an existing backup to your Google account

1. Open Xperia Companion on your computer and select Xperia Transfer → Next.
2. Select source device Sony → Next.
3. Select Select backup → Next.
4. Choose your backup file → Next. 
5. Enter password if required → OK.
6. Uncheck all check boxes except Contacts → Next.
7. Select Xperia + Google™ → Next.
8. Follow the on-screen instructions to log in to your Google account.  

The contacts will be transferred to your Google account, and you can view them at https://contacts.google.com/.