Are you referring to adding items to a drop down list when you are designing the form, or at runtime when a user is filling in the form?
If it's design time, as far as I know you must manually add the items.
Yes, its at the design time.
lets consider the scenario like this, i had 10 items when i initially designed the PDF form. But later i got a new requirement that the dropdown needs to be updated with 90 more items. so do i need to add those 90 items manually or is there any other way i can add those 90 at a time. something like copy paste or something.
You will need to manually add the items to the drop list. Once the list is complete, you can add the configure drop down list object to yuor custom library so it can be used in the design of other form templates if desired.
one dirty solution for your need,
<items save="1" presence="hidden">
look at the above xml snippet. this is how your drop-down values list will be represented.
So you could open the XML Source in the LiveCycle designer and paste your 90 values here.
if you have only the raw values like value3, value4 etc, you can make use of the TextPad software to enclose them with the angle brackets (<>) using regular expression replace method.
Hope it will help you.