There is no official support for that in projectile (I think) but, you can use dir-local-variables and a little bit of hacking code like this:
(defun open-local-file-projectile (directory)
"Helm action function, open projectile file within DIRECTORY
specify by the keyword projectile-default-file define in
(let ((default-file (...
To look within a particular project, I often use ag. It is a front end for The Silver Searcher which allows you to recursively search a directory for a regexp in a file or filename. The ag package presents a nice list of matches with links to their source. Similar packages exist for grep ((emacs) Grep Searching or via C-h i d m emacs <RET> m Grep ...
You can also try C-u M-x project-find-regexp.
It's a part of the project package, built-in in the latest versions of Emacs but also distributed through GNU ELPA.
Be warned though: this calling convention doesn't honor the .gitignore entries (only ignores standard "junk" files and utility directories). So if you have a lot of files in the ...