This happened to me too. I am using Windows 10 and would like to change the icons back.
The CCleaner Browser is only being offered to a limited number of users when Updating CCleaner itself.
Most browsers will change your file associations when you first install them.
If/when this happens then it's easy enough to change them back.
For the example of PDF files:
Right click on any PDF file and select 'Open With > ' and then 'Choose another app'.
Choose the application that you want to open PDFs with, then tick "Always use this App to open PDF files'.
The file will open in the App that you have chosen, you can close it if you dont want to work on it just then.
Job done, and your PDF icons should change back to how they were. (If they don't change straight away then restart your computer).
A change has been promised to the Browser to make file associations a more obvious seting during the Browser install.
The Browser is now available for others who don't get the offer during a CCleaner update/install.