Tables should ignore fixed proffered width with the value of 0, this is the default behavior in Word as well.
Workaround (when converting Flow to PDF documents):
foreach (var item in tables)
{
if (item.PreferredWidth.Type == Telerik.Windows.Documents.Flow.Model.Styles.TableWidthUnitType.Fixed &&
item.PreferredWidth.Value == 0)
{
item.PreferredWidth = new Telerik.Windows.Documents.Flow.Model.Styles.TableWidthUnit(Telerik.Windows.Documents.Flow.Model.Styles.TableWidthUnitType.Percent, 100);
}
}