How to open multiple windows of Foxit Reader on mac?
According to this Foxit for Mac help page, you can open multiple instances.

Read PDF Files in Multiple Instances

By default, Foxit Reader displays multiple PDF files in different tabs in a single instance. However, you can enable multiple
instances mode to view PDF files in multiple instances when
double-click to open PDF files, which is ideal for reading PDF side by
side.
To enable multiple instances mode, go to Foxit Reader &gt; Preferences &gt; Documents, check the Allow multiple instances option in Open
Setting group, and click OK to apply the setting.
While in multiple instances mode, you can revert to single instance
mode by redoing the steps and unchecking the Allow multiple instances
option.