@dzigner: Where an exported PDF file is stored depends on your settings.
The default is that the PDF file is stored in the corresponding notebook directory, which is within the app directory. The default app directory follows an Android standard and is somewhat hidden, it is usually Android/data/com.acadoid.lecturenotes/files, starting in the root of the SD card; you can set a custom app directory in the app's storage settings (when doing that, note that it takes an app restart for such change to take effect and that your notebooks are not automatically moved to the new location, but the latter can be easily achieved by a backup of the notebooks board at the old location and a restore at the new location).
Besides that, you can also set a PDF directory in the app's storage settings, then all exported PDF files go there.
In case you prefer to share exported PDF instead of viewing it, change the options in the app's export settings.