Every conversion on Sourceback ends with a number out of 100, per page and for the whole file, before you're asked for anything. People read it as a grade. It's closer to a measurement, and knowing what is being measured tells you when to trust it, when to ignore it, and when it's telling you to walk away.
After the tool rebuilds a page into its editable form, it renders that rebuilt page back to pixels and lays it over a render of the original. The score is how closely the two match — where the ink is, how much of it there is, whether the shapes and text blocks land where they should. It is a comparison of pictures, done the same way for every file, with no judgement about whether the page is "good".
That has two consequences worth holding onto. A high score means the rebuilt page looks like the original, which is what you want but not everything you want: it doesn't check that a word is spelled the same or that a paragraph reflows well when you edit it. And a lower score sometimes reflects a rendering difference rather than lost content — a font substituted at preview time, a gradient captured as a flat fill — so the side-by-side preview underneath the number is there to be looked at, not just scrolled past.
The number tells you how much came back. The side-by-side tells you what didn't. Read them together.
There is a second safety net underneath the bands. If a page clearly has content but the rebuild recovered only a small fraction of its ink, it is warned or declined regardless of what the pixel comparison says, so a near-empty page never ships quietly as a "pass".
Almost entirely how the file was made, not which tool opens it. A clean export from InDesign, Illustrator or PowerPoint scores high because everything on the page still exists as objects. A print-ready PDF with transparency flattened scores lower because the flattener turned overlapping objects into image slices. A PDF that was itself made from a scan or a screenshot scores zero and is declined.
Two things you can do about a low score before giving up: find an earlier or different export of the same document (the version sent for proofing is often less flattened than the one sent to print), and check whether the file is a scan at all — the two-minute test in Flattened, scanned, or fine? covers that.
The score is shown before payment on purpose. As a rule of thumb: pay at 80 and above without much thought; at 45–80 pay if the preview shows the parts you care about came back; below 45, ask whether the time you'd spend repairing the rebuild is really less than redrawing a page that simple. For a dense brochure spread it usually is. For a one-line flyer it usually isn't, and no converter should pretend otherwise.