Hi Rajiv,
If you want to know who all has downloaded a particular asset, like a white paper, you wouldn't usually be looking at the lead source. Lead source is normally set to keep the original value and not over-write it. So you may have someone with a lead source of web visit that later downloads the white paper, their source would still be web visit (you may want to check your field management in the admin section to see if yours is configured this way).
The best way to track on a micro level is to make a program for the asset. So you'd have a program in Marketing Activities specifically for this white paper. Here you could also create a SFDC campaign that it syncs to. This way you can see all the leads who downloaded that asset, you'd need to create smart campaigns to change their statuses to download or engage or whatever you're using based on the action of filling out that particular form, etc. If you are looking at tracking code, you can include that utm_source in the filter, there is criteria you can add called "query string" and I'd select contains as the filter.
Hope this makes sense! There's many ways to track and report in Marketo so you just need to figure out if you want to look at high level or micro level data.
Kim
.png)