What an MQL5 demo download actually gives you
Every paid Expert Advisor on the MQL5 Market carries a free demo. It is the full algorithm with one restriction, and knowing exactly what that restriction is tells you what the paid version is actually for.
The demo is not a cut-down build
Every listing on the MQL5 Market carries a demo you can download at no cost, and it is the same algorithm the paid version runs. Nothing is disabled inside the logic, no features are held back, and the entry and exit rules are the ones you are paying for.
There is exactly one restriction: the demo runs only in the Strategy Tester. It will not trade a live account, and — the part that surprises people — it will not trade a demo account either. A broker demo account and the MetaTrader Strategy Tester are different things, and the free build works in the second but not the first.
What the Strategy Tester will tell you
A great deal, and for nothing. You can run the algorithm over your own broker's price history, with your own spread, your own symbol and your own account size, and watch what it does. That is the single most useful thing you can do before buying, because it is the only test whose assumptions you control.
It settles the questions a sales page cannot. How often does it trade. How long does it hold. How large do the positions get after a loss. Whether it ever has no stop on the market. None of those depend on the broker being perfectly modelled, and all of them are things a seller has no reason to volunteer.
And what it will not
A tester result is still a simulation. It models spread, slippage and execution from historical data, and real money meets none of those exactly — the gap is widest for strategies that hold for minutes, where a fraction of a pip of slippage per trade is the whole edge.
So treat a tester run as a description of the strategy rather than a forecast of the return. What it does is knowable; what it will earn is not.
A paid product offered as a free live download is pirated
This is worth stating plainly, because the search term brings people to exactly these downloads. If a site offers the paid build — the one that trades live — at no cost, it did not come from the seller.
Such builds are usually modified, always unsupported, and a common way to lose an account. You are handing trading authority over real money to a binary that somebody altered for reasons they have not told you, and there is no version to update to when it breaks. The free demo from the MQL5 listing is the legitimate free download, and it is genuinely free.