Generally speaking, leads are usually a short-lived type of record. That is, most leads should either be dropped (cold leads) or converted (hot leads) within a relatively short period of time, even if your sales cycle measures in months. Since salesforce prompts users to merge leads into existing matching accounts and contacts, there's usually no problem having one person be more than one lead in the system; each represents a single potential opportunity for a closed sale.
So, as an example, for your use case, your sales process might look like this:
- A potential client downloads something.
- A lead is created.
- Sales checks for duplicate leads, and merges them.
- Sales follows up with the lead.
- After determining this is a qualified lead, converts to opportunity.
- Follows the sales process on the opportunity.
- Closes the opportunity.
Every lead would go through this process, even if it's the same person. The only tricky part is how to keep multiple sales associates from speaking with the same contact when more than one lead is generated at the same time. Usually, it's as simple as using the Find Duplicates button, or setting up duplicate rules to avoid duplicate active leads. Then again, some organizations don't worry about duplicate leads, because they're handled by different people (e.g. one lead goes to Bob because he works hardware sales, while another lead goes to Alice because she works software sales).
If you still need additional guidance, I would strongly suggest you find a competent consultant, sit down with them for an hour or three, have them understand your business model, and then have them help you tailor the system to suit your business' sales model. The standard out-of-the-box configuration for leads is pretty typical for a given organization, but you might have special requirements that might not be suitable for an answer on this site. Having a solid understanding of a sales cycle, and knowing how to configure that cycle in salesforce, is a cornerstone of how a business can effectively increase their sales and decrease their costs by using Salesforce.