Some PDF files may be difficult to import into LiveCycle Designer ES. Conversion problems can depend both on the application used to produce the PDF file and on choices made when the source document was authored. For example, these original sources of PDF files have proven difficult to import into LiveCycle Designer ES:
When an image in a PDF form contains a large number of lines, such as a vector-based image, LiveCycle Designer ES needs a significant amount of time to process the lines during an import operation. For faster processing, remove the image from the source file that is used to create the PDF. If required, you can reinsert the image after you save the form design in LiveCycle Designer ES. If the source file is not available, deleting the image from the form in LiveCycle Designer ES will improve performance of the form.
If you are having conversion problems and you have access to the source files for the original form, try re-creating the PDF by using Distiller. If the original PDF was not created correctly, re-creating it may resolve the issue.
See also 

About reducing PDF conversion problems