If you want to retain any of the Solution Centers functionality then you must look to 3rd party applications on this printer model or upgrade to a model with supported apps on Win8.1.
For scan to PDF you will need an Adobe product. Scanning to PDF is proprietary so only licenced adobe products can perform that task, unlike scan to TIFF which is universally free. Otherwise you can scan to Tiff and use a free Adobe Converter posted all over the internet to get it into that file type.
For OCR you will need an OCR application. I dont have any suggestions for you here but if you search around Im sure you can find one.
HPToolbox is not for scanning. HPToolbox is for maintaining a hardware status report on your pritner and retreving information about the printer and cartridges.