Of late, Google has been constantly improving PDF Viewer in the Chrome browser. For instance, it developed an option to see PDF in a two-page view and let you save PDF forms. Now, the company is working on a new UI for the PDF Viewer.

Currently, Google Chrome displays this information, and these options for a PDF loaded in the browser when you hover over with the mouse.
The toolbar at the top of the page shows PDF name, Page numbers, Rotate clockwise, Print and download buttons.
On the right sidebar, PDF Viewer shows Fit to page and Zoom In/out buttons.

The PDF Viewer’s new toolbar UI to consolidate and display all the following PDF controls in one place.
Sidebar nav, PDF title, Page numbers, Zoom controls, Fit to Page, Rotate, Download, and Print buttons along with additional menu at the end that shows “Single Page View” and “Two Page View” options when clicked.
It’s worth noting that options to Rotate Page in clockwise (Ctrl ]) and counterclockwise (Ctrl +]) still available on PDF right-click menu.
Enable the new UI for PDF Viewer in Chrome
1. Visit chrome://flags page
2. Search for “PDF Viewer Update”

The flag description says “the PDF Viewer will display an updated UI with new options and features” on Windows, Mac, Linux, and Chrome OS.
3. Select “Enabled” from the dropdown and restart the browser
Update July 7, 2020: The Chrome PDF Viewer update with new toolbar is now live and functional in Canary.
Recently, Microsoft announced it will bring Table of Contents, Highlight mode, Text notes, digital signatures, Protected PDF files, and other new features and improvements to the new Microsoft Edge PDF Reader.
Mozilla hasn’t left behind either. The company constantly updates PDF.js. Recently, a lot of improvements landed in Firefox Nightly related to the PDF viewer.
For instance, you can now set Firefox as default PDF Reader on Windows 10 and the browser now shows PDF opened from it with Edge in app mode.
Related articles:
Chrome gets Two-Page View for PDFs, here is how to enable it
Chrome lets you download Edited PDFs now
Google Chrome: How to Always Open PDFs with System Viewer?
Google Chrome to Open Downloaded PDF Files in It by Default