Swift snippet for jumping to a specific PDF page in WKWebView.

ViewController.swift

    func webViewGotoPDFPage(index: Int) {
        if (index < 1) { return; }

        let pageCount = totalPDFPages(filePath: webViewLastURL!);
        let pageIndex = max(min(index, pageCount), 1)-1;

        /*
        let firstPage = self.webView.scrollView.subviews[2].subviews[0];
        //let contentHeight = self.webView.scrollView.contentSize.height;
        //let totalPaddingSize = CGFloat(PDF_INNER_PADDING)*(CGFloat(pageCount)-1) + CGFloat(PDF_OUTER_PADDING)*2;
        //let averagePageHeight = (contentHeight-totalPaddingSize)/CGFloat(pageCount);
        let firstPageHeight = firstPage.bounds.size.height;
        let scrollY = (firstPageHeight*CGFloat(pageIndex)) + (CGFloat(PDF_INNER_PADDING)*CGFloat(pageIndex)) + CGFloat(PDF_OUTER_PADDING);

        self.webView.scrollView.setContentOffset(CGPoint.init(x: 0, y: scrollY), animated: animated);
        */

        var pdfView = self.webView.scrollView.subviews[2];
        var scrollY:CGFloat = 0.0;
        for i in 0..<pageIndex {
            for subview in pdfView.subviews {
                var y2 = subview.frame.origin.y + subview.frame.size.height;
                if (y2 > scrollY) {
                    scrollY = y2;
                    break;
                }
                self.webView.scrollView.setContentOffset(CGPoint.init(x: 0, y: scrollY), animated: false);
            }
        }
    }

    func totalPDFPages(filePath: URL) -> Int {
        let doc = CGPDFDocument.init(filePath as CFURL);
        return (doc?.numberOfPages)!;
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        webView.isHidden = false;
        if (webViewStartPDFPageIndex > 1 && webViewLastURL?.pathExtension.lowercased() == "pdf") {
            webViewGotoPDFPage(index: webViewStartPDFPageIndex);
        }
    } 

For anyone stumbling upon this - it seems a better way to get page dimensions is using CGPDFDocument, CGPDFPage and getBoxRect() - see this discussion: https://stackoverflow.com/questions/3045587/how-to-get-actual-pdf-page-size-in-ipad

Also note that if you're targeting iOS 11+, then you can simply avoid using web views altogether and use the new PDFView instead. It has functions such as goToNextPage and goToPreviousPage.