It seems to be a problem with "docController", is a local variable and, i think, must be a instance variable.
Change
UIDocumentInteractionController *docController = [UIDocumentInteractionController interactionControllerWithURL:[NSURL fileURLWithPath:filePath]];
for
docController = [UIDocumentInteractionController interactionControllerWithURL:[NSURL fileURLWithPath:filePath]];
and in your .h declare
UIDocumentInteractionController *docController;