Which product/version (including minor)/platform/OS and its version?
Does this happen with any PDF, including a blank one, or with [some] PDF that you received from someone else? One of the possible reasons for this is that your PDF contains signature fields (possibly unsigned and possibly hidden). For instance, if your PDF is Reader-enabled then it contains a hidden digital signature that controls the Reader rights. The other possibility is that PDF is certified with a possibly invisible signature. If PDF is Reader-enabled, open it in Adobe Reader and the top icon in the left-hand panel opens when clicked the "Document Extensions" pane. If you have this icon, PDF is Reader-enabled and you cannot place e-signature in it. If PDF is certified, when you open it in Adobe Reader, the Document Message Bar at the top of the window tells you that this PDF is certified and the certification state.