We have tried multiple PDFs and all seem to have this issue.
When a user signs a document then selects that signature and changes its opacity, the app crashes. This happens when the opacity is not 5% or 100%.
We loaded a PDF into a PdfLoadedDocument, changed a few textbox values and then pass it to the SfPdfViewer to be viewed. Select a signature field, sign it and click 'done'. Now, re-select it and change it's opacity. Crash.