Actually the io.elementary.code works as expected. You just need to open a folder inside it which is called a workspace/project
Try the following command for elementary OS 5.1 Hera
So to answer your question Look at the bottom left corner of the screenshot I attached. That's the open Project button
Also, make sure the sidebar is ...
If it is only about rotation of the pdf, it can be easily done by Evince, the default Document viewer for Elementary Os, I do not know about the default version, but I did upgrade it with the flatpak version and rotation is easy done with not notable quality loss, at least i do not notice any.