Most of the PDF standard metadata properties are linked (hard-coded in the tools) to Windows property system default properties, e.g.
System.Title for the Title field, etc. These Windows default properties can't be configured, so there is no way to change how these properties are displayed or edited.
If I understand the property system specification, it would be possible to achieve this if the
relatedProperty element, of the property specification schema, worked but I never managed to use it (I suspect it is not implemented).
If you really need it, there is a way but is not totally perfect. You need to create a custom property with the same name as the PDF metadata property, configure it as you need, e.g. Display type set as enumerated and Editor control set as Drop list, and then map (from the mapping tab) the standard property to this new custom property.
Example: For the Title property, create a custom property with the "Label" column set as "My Title" and the "Name" column set as "Title" (this is the one that needs to be equal to the name used in the PDF to store the data) and configure it as needed from the advanced settings editor. Now, from the Mapping tab, map the Title property to the "My Title (PDFShellTools.PDF.Title)" one. Configure the properties to show in the "Details pane items" tab, and apply the changes. You will now be able to edit the PDFs Title property using a drop list.
As you can deduce, this is emulating what the above referred Windows property system relatedProperty setting would do, if we managed to use it.
The problem with this method is that the System.Title property no longer links to the PDFs Title metadata, i.e. to show the PDFs Titles in a Explorer column you now need to use your custom "My Title" column. You can no longer use the same column to show/sort/search the Title aggregated with other file types, e.g. .docx.
