PDF is a great way to create static content for tablet and mobile devices. Specle is working to develop new standards to support these devices.
The most important thing to note is that PDF/X-1a:2001 is not the format to use for creating tablet ads. Because transparency is flattened, it can cause white lines to appear in your ad.
The basic rules to follow are:Create everything in RGB. Use RGB colours in InDesign, and save all your images using the sRGB profile
Make your file in InDesign