For everybody still looking as of 2025 I just worked out a way to do it reliably:

        public static float Round(this float value, float step = 1f)
        {
            if (step <= 0) throw new System.ArgumentException("Step must be greater than zero.");

            float remainder = (value = Mathf.Ceil(value / 0.001f) * 0.001f) % step;
            float halfStep = step / 2f;

            return
                remainder >= halfStep ? value + step - remainder :
                remainder < -halfStep ? value - step - remainder :
                value - remainder;
        }

        public static string[] GetLines(this PdfDocument document, int page, char wordSeperator = ' ', float tolerance = 0.01f)
        {
            var lines = new List<string>();

            if (page > 0 && document.NumberOfPages >= page)
            {
                var wordGroups = document.GetPage(page)
                    .GetWords()
                    .GroupBy(w => ((float)(w.BoundingBox.Top + w.BoundingBox.Bottom) * 0.5f).Round(tolerance));

                foreach (var group in wordGroups.OrderByDescending(g => g.Key)) // top to bottom
                {
                    var line = string.Join(wordSeperator, group.OrderBy(w => w.BoundingBox.Left));

                    if (line.NotEmpty()) lines.Add(line);
                }
            }

            return [.. lines];
        }

Enjoy. Also take a look at the Cutulu SDK by @narrenschlag on GitHub and leave a star if you liked this. :)

~ Max, der Narr