iTextSharp.LGPLv2.Core 1.7.5 was not good for this in my use case; neither was Spire.PDF.

However, PdfPig worked a treat: https://www.nuget.org/packages/PdfPig

var text = "";
var doc = UglyToad.PdfPig.PdfDocument.Open(bytes);
foreach (var page in doc.GetPages())
{
    text += page.Text;
}

via https://stackoverflow.com/a/60501282