ODT is one of the options for the text file format, which was in little demand and gained relative distribution only in 2014, when Google Docs began to use the ODF format. It takes its name from the OpenDocument Text Document, and is used in office programs that are based on utilities like Open Office or LibreOffice. The open standard ODF (OpenDocument Format) was created by the OASIS community and was intended for the free creation, editing, storage and exchange of office documents. Within its framework, the ODT format also allows the use of databases, presentations, images, tables in text files.
PDF is short for Portable Document Format, which can be translated from English as “Portable Document Format”. It was developed by Adobe Systems for use by US federal authorities as a tool for storing working documents. This is a universal cross-platform format, which is now standard for electronic documents. It serves to convert text files (including photographs or other images) into electronic documents without any loss. To read PDF files you need special programs - Adobe (Acrobat) Reader, PDF-Viewer and others.