Parhaps I found a new workaround which works in Cubase 9.0.10. It enables you to open the pdf manual locally in the internet browser. Honestly, I’d like to use the pdf reader app but I don’t know how to do that. And so, simply creating a shortcut in the Desktop is still the best way for me 
1, Download the pdf manual from “https://steinberg.help/”.
2, Open “C:\Program Files\Steinberg\Cubase 9”, create a folder named “Documentation” and unzip the pdf manual there.
3, Open “C:\Program Files\Steinberg\Cubase 9\Help”.
4, Copy “help-urls.xml” and paste it in the Desktop.
5, Rename the original one “help-urls.xml.old” for backup.
6, Rename the copied one “help-urls.txt” and open it in the text editor.
7, Find your Cubase system language and edit the url like this. The location is the same but the file name is up to your language so please refer to the pdf manual’s name you downloaded.
BEFORE
AFTER
8, Save and exit.
9, Rename it “help-urls.xml” and move it to “C:\Program Files\Steinberg\Cubase 9\Help”.
10, Start Cubase and press F1. Now you can access User Manuals locally in the browser.
*If something bad happens, please delete the “help-urls.xml” you’ve edited and rename the “help-urls.xml.old” as it was before.