let xml = export_xfdf(&doc, true, true, "form.pdf");   // comments and/or field values
let fdf = export_fdf(&doc, false, true, "form.pdf");
let txt = export_data(&doc, Format::Txt);               // or Xml, Csv
let report = import(&mut doc, &bytes)?;                 // XFDF, FDF, XML, CSV or text, detected

XFDF (ISO 19444-1): every comment type PdfCraft knows (text, free text, line, square, circle, polygon, polyline, markup, stamp, caret, ink, attachments, sound, redact) with rect, name, author, subject, dates, flags, colours, opacity, width, icon, quads, line ends, ink, default appearance, callouts (callout, fringe, intent, head), pop-ups and reply threads (inreplyto); field values nested by name.
FDF: fields (with /Kids hierarchies) and comments as direct objects.
Form data: Acrobat's XML (xfdf:original keeps names that aren't XML names), CSV and tab-delimited text (a row of names, a row of values).

Import replaces comments with the same /NM (and drops the replaced comment's pop-up), draws appearances PdfCraft can draw, and sets values through the form's own checks (formats, validation, recalculation); read-only fields take imported values, as in Acrobat. XML is read with roxmltree.