Edith, you didn't give your grandfather's name, but thousands of names from the surviving "Unburnt Documents" can be read on familysearch.com on the LDS Library Catalog, and then the appropriate film can be ordered to view locally. Search the Catalog online for film number 2028521. These are the records from the PRO Kew which were mentioned by James McMenemy in answer to your question.
They are described as:
Title: Military records of the British Army, 1914-1920, 1914-1920
Microfilm of original records in possession of the Public Record Office, London, England.
These records are known as the "Unburnt documents". They represent about 8 percent of the original file. These are files that were submitted to the Ministry of Pensions. They were undamaged during World War II when the main file was 70 per cent destroyed in the Blitz.
Contains service files of soldiers who were discharged from the British Army from 1914 to 1920. The files are organized alphabetically by surname and given name. Some soldiers may have enlisted as early as the 1890s. Includes unit name and number, birthplace, age at time of enlistment and name and address of next of kin. May also include names of parents, spouses and children. May also note occupation prior to military service and assignments in the Army. Includes date of discharge and reason for discharge. Death certificates of some pensioners are included up to as late as the 1950s.
Public Record Office No.: WO 361
Publication Salt Lake City : Filmed by the Genealogical Society of Utah, 1990-1995 on 4835 microfilm reels ; 35 mm. & 16 mm.
Sue