So I had to review a huge PR today and I couldn't see all the files from top to bottom and I though maybe there is a shortcut to navigate between files. Nope. All I can do is show hide left and right sidebar. 
This appear at the end of the first file :
To view another file, use the file tree in the right sidebar or navigation buttons in the header.

Ok but can I use the navigation without a mouse ? What is annoying in this context is that there isn't a file view... So you must navigate from previous/next file until you get the right file. With 131 files changed, it's inefficient navigating in all those files... I can't imagine what reviewing a PR double that size could be... 

Anyway, to summaries in bullet point :

• Navigating through keyboard in Pull Request would help a lot.
  • Next/Previous file.
  • Next/Previous hunk.
  • Collapse/Display current hunk.
  • Navigate lines.
  • Add comment for the line/files.
• Having a file explorer as sidebar would help review/navigate to a specific file when needed.

I have use Azure DevOps for the past 3 years. I can said that navigating in Bitbucket is not as easy nor intuitive. Maybe I'm not used to the workflow, but I definitely think those 2 bullets points are a must have that is missing.