when ever i am trying to open document following error is coming “File format error found at
SAXParseException: '[ line 2]: Couldn’t find end of Start Tag font
', Stream ‘’, Line 2, Column 989(row,col).”
The suggestions of the tutorial cited by @Grantler might be useful - well, some of them are outdated (LO opens the problematic files since 5.4 - no worse than AOO, because the error makes some parts of the file to be skipped). But the most important is to report the specifics on how the problem started - i.e., when LO generates those corrupt files. Doing that allows us to find and fix problems - like in tdf#113790.
Or, if you are not comfortable fixing it yourself, upload your document to a file sharing service such as Mediafire and add a link in your question; someone else may be able to help.