Digital libraries have been built all over the world. One of the main challenges in maintaining digital libraries is the digital preservation. The aim of digital preservation is to ensure that digital records are filed and are made available throughout time. Each digital library consists of different documents with different file formats. Due to rapid obsolescence in hardware and software technology it is necessary for the libraries to look into the details of the file formats. Thus the file format for long-term preservation plays a very important role during the practices of digital preservation. The article tries to analyzing the current situation of the file formats, then points out the standards in choosing the file format for long-term preservation based on generalizing the researches by scholars. After comparing the PDF and XML, the author thinks the XML file format is more suitable for long-term preservation.