There can be various reasons why PayPal withholds funds when completing a purchase. One of the most common is for anti-fraud checks, such as payments made from locations which differ from your normal behavior.
Attempting to pay with eCheque/ACH can also result in a delay because PayPal will hold the funds until they are received from your payment source. All of this is automatic and beyond our control. Once things are cleared, the download link is automatically sent to your email and the product becomes available to download.