This property determines if the NeedAppearances flag in the PDF
is set when field values are changed.
The NeedAppearances flag signals to the viewing application that
field appearances should be automatically generated rather than
using the appearance embedded in the PDF.
If this property is true, every time you change the value of a
field ABCpdf will ensure that a NeedsAppearances flag is available
and that it is set to true. It only ever sets the flag to true, it
will not set it to false. It only sets the flag when field values
are changed, it will not set it if no field values are changed.
This means that if you read a document in which this flag may
already be set to true, to revert it back to false, you would need
code of the folllowing form.
var cat = doc.ObjectSoup.Catalog;
var form = cat.Resolve(Atom.GetItem(cat.Atom, "AcroForm"));
if (form != null)
Dim cat = doc.ObjectSoup.Catalog
Dim form = cat.Resolve(Atom.GetItem(cat.Atom, "AcroForm"))
If form IsNot Nothing Then Atom.RemoveItem(form, "NeedAppearances")
PDF 2.0 Related Changes. In PDF 2.0 the 'true' setting
for this property has been deprecated.
The logic is that documents should generate their own appearance
streams rather than relying on the behavior of the viewer. There is
truth in this, but also a problem - the generation of appearance
streams is not defined in the specification - it inevitably varies
So the reality is that, without this flag, users may see a
document which looks subtly different from the one they expect.
Currently we leave this default as true so that users see the
documents as they expect. If you require strict adherence to PDF
2.0 then you may wish to set it to false.
However since PDF 2.0 has deprecated this setting, the default
value for this property is likely to change in the future. As such,
if you are relying on a particular value you should set the
property explicitly in your code.