Ctrl + Shift + F as others have stated. In addition, toggle those four buttons below the search bar to vary the scope of your search... In ProjectModuleDirectoryScope. You really have to watch out. Also, take note of the check boxes above. You want everything to be properly set. You don't want to be searching a Directory when your intention is to search the whole project.
There are multiple ways to do this. I am using PyCharm 2019.1, but I bet that it will work in previous versions.
Find in path shortcut:
⇧ + ⌘ + F (mac) or
Ctrl + ⇧ + F (windows or linux)
This will open the following in which you can type the word you are looking for.
Find action shortcut
⇧ + ⌘ + A (mac) or
Ctrl + ⇧ + A (windows or linux)
The following will be opened in which you could type Find, and select the option Find in Path (notice that the command for this action is displayed next to it). After selecting this, you will be taken to the same menu that is opened when running the Find in path shortcut.
Right click on your project's directory, and choosing Find in Path...
You can right-click on your project's name, and select the option Find in Path... which will also open the menu that is opened after running the Find in Path shortcut.