Carol N, I have been in the process of converting paper Court Forms to electronic fillable forms. This has also been extended to include retro fitting electronic forms our printers have created (and kept as PDFs) during their printing process. I hope I understand your question.
If you are starting with a file (ANYFILE.pdf), normally when you open it from the folder it's kept in, it will open using the ADOBE READER application. On one of the tool bars, there is an ICON that looks like two documents on top of each other and when your pointer hovers over it it indicates, "Form Tasks". when you click on this button, a drop-down menu will appear. The top three options are what you are interested in using. the top one is, "Make form fillable in designer..." and the second is, "Edit fillable form..." and the third is, "Create new form..." Selecting any of these will execute ADOBE DESIGNER.
If the document you are viewing using the reader is the document you want to use, and has no fillable fields, select the top option. If the document you are viewing using the reader is the is the document you want to use and already has fillable fields, select the second option. Once a selection has been made, you will go through a series of notifications and menus regarding this process. read through them to determine which is most appropriate for what you need. When completed ADOBE DESIGNER will open with your document in an editable format ready for you to add whatever fields you require.
You could also get to this point by starting the ADOBE DESIGNER application and opting to OPEN the PDF you need (just as you would from MS WORD). Another is by right clicking on the file as opposed to double clicking on it. This allows you the choice of what application to use when opening the file (.pdf) by selecting the option, "OPEN WITH..."
Now that you have the document open in ADOBE DESIGNER, there are a whole number of other issues you may have as to the fillable objects you use.
I hope this was helpful. If not could you be more specific.
Ken K