I have quite few different print composers. Can I somehow export them to pdf, one pdf for each composer, at the same time?
To take it even further, you can export multiple print composers from multiple .qgs files using the method from "Batch export from multiple QGS projects to image or PDF" which uses pyqgis and Maps Printer.
For example, using Maps Printer:
Open the Maps Printer window from Plugins > Maps Printer > Export multiple print composers
Then set your options as needed like selecting an output folder, the print composers to export and output file type. Click Export when ready.