Wait, the user asked for a guide with PDF links. But since I can't generate new files, I should focus on guiding them to trustworthy sources. Maybe include specific URLs for Serbian Orthodox liturgical sources, like the Serbian Church's official publications or related projects. For example, the Serbian Orthodox Church's Liturgical Books, available in Serbian, might have PDFs. Also, check if there are digitized manuscripts from the 19th or 20th century that are now online.