The "Download Now" link directs you to the iTunes App Store, where you must continue the download process. You must have iTunes installed in order to open the link, and you must have an active iTunes account to download the application. This download may not be available in some countries.
From Infield Sales Pro:
MobilePro PDF - App DescriptionUsers of Salesforce.com Enterprise, Unlimited and Developer editions can access their PDF documents in their Salesforce Documents folder on their iPad, even without a Wi-Fi or 3G data connection. MobilePro PDF, a PDF viewer for the iPad, allows Salesforce.com users to easily upload PDF documents from Salesforce.com's Documents tab directly to their iPads. This app provides instant access to critical PDF documents on their iPads while meeting clients in the field. When online, users can download PDFs and also e-mail them to clients and colleagues. When offline, they can access those saved documents without spending extra time or bandwidth to retrieve them. Thanks to MobilePro PDF, Salesforce.com users can augment their sales presentations, address client concerns more effectively, and close deals more efficiently. Additional key features: View, download, and synchronize pertinent PDF documents from Salesforce.com's Documents (Not the Content section, but the Document Tab) Synchronize the Documents folders organization of your Salesforce.com instance A Delete feature that removes PDFs that are deleted in Salesforce, but retains PDFs that are deleted only from the app Reverse-pinch and pinch gestures to zoom in and out of your PDF documents. Compatible with both iPad 1 and 2 Compatible only with PDF files (each file can be up to 5MB in size). Recommend you save PDF documents to your iPads when connected to Wi-Fi so its faster and it doesnt use any 3G Data plan