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There's no time limit for requesting your PIP report, but there may be practical issues depending on how long ago the Decision was made as the DWP move files to offsite storage.
Start by contacting the DWP office dealing with your PIP claim and make the request, they should be able to tell you whether they can do it or not.
If they can't then you need to make a Subject Access Request.
SAR :
www.dwp.gov.uk/docs/request-for-personal-information.rtf From this page :
www.dwp.gov.uk/privacy-policy/data-protection/ Gordon