Portable Document Format (PDF) is a type of document created by Adobe back in 1990s. The purpose of this file format was to introduce a standard for representation of documents and other reference material in a format that is independent of application software, hardware as well as Operating System. PDF files can be opened in Adobe Acrobat Reader/Writer as well in most modern browsers like Chrome, Safari, Firefox via extensions/plug-ins.
ODT files are type of documents created with word processing applications that are based on OpenDocument Text File format. These are created with word processor applications such as free OpenOffice Writer and can hold content such as text, images, objects and styles. The ODT file is to Writer word processor what the DOCX is to Microsoft Word. Several applications including Google Docs and Google's web-based word processor included with Google Drive can open the ODT files for editing.
You can also convert PDF into many other file formats. Please see the complete list below.