Test cases with Aspects! https://github.com/steipete/Aspects

TestCaseWithAspects.m

- (void)testRelativeURLs {
    PSPDFDocument *document = [[PSPDFTestAssetLoader new] documentWithName:@"Testcase-relative-links.pdf"];
    NSArray *annotations = [document annotationsForPage:0 type:PSPDFAnnotationTypeLink];
    PSPDFLinkAnnotation *link = annotations[0];
    NSString *const referenceString = @"Testcase-relative-links/Simple.txt";
    XCTAssertEqualObjects(link.URL.absoluteString, referenceString, @"URL should be correct");

    PSPDFViewController *pdfController = [[PSPDFViewController alloc] initWithDocument:document];

    __block NSURL *showPreviewURL = nil;
    [pdfController aspect_hookSelector:@selector(showPreviewForURL:animated:title:options:sender:) withOptions:AspectOptionAutomaticRemoval usingBlock:^(id instance, NSArray *args) {
        showPreviewURL = args.firstObject;
    } error:NULL];

    [pdfController executePDFAction:link.action inTargetRect:CGRectZero forPage:0 animated:NO actionContainer:link];

    NSURL *referenceURL = [document.fileURL.URLByDeletingLastPathComponent URLByAppendingPathComponent:referenceString];
    XCTAssertEqualObjects(showPreviewURL, referenceURL, @"URL must be correct");
}