Under Options > Configure TeXstudio > Commands
I changed PdfLateX to use the -output-directory=tex
option. I checked Show Advanced Options
in the bottom right corner and set all additional search paths under Build
to the used directory name.
Everything works (I can compile to PDF and preview it in the internal PDF viewer) but I don't know how I should change the External PDF Viewer
command so that it opens the PDF from the subdirectory.