Path for saving PDF in TexStudio

As stated in the comments above, TXS has no direct control on the placements of the PDF. That is determined by pdflatex and its options (which vary depending on the TeX distribution you use); if you use Miktex see @Diaas answer below. Otherwise, you must enable TXS to copy the .pdf in your desired folder after each compilation: you can create a user command and run it manually (step 1), or to run it automatically after each pdflatex compilation (steps 1+2).

1. Create a user command at Options -> Configure TexStudio -> Build -> User Commands

On Windows

Command name (copypdf) and display name ('Copy PDF') are separated by a colon. The call executes a windows shell (cmd) and runs the copy command within

On Unix systems (e.g. Linux, OSx)

Write cp ./%.pdf /targetdir/ as shell command.

As a result, you'll have a new menu entry Tools -> User -> Copy PDF.

You can run it any time you like (and also assign a shortcut to it in the options).

2. If you want an automatic solution and not care about copying the pdf at all, just extend the command of the default compiler in the following way (Note: You need to have the advanced options switched on - checkbox in bottom left corner):

This calls the internal copypdf commad, which we've just definded above, after every pdflatex run. Of course, you can replace the simple copy operation by a shell script if you need additional logic (e.g. determine the target dir depending on the source dir).