When opening a saved (or blank) document, you are unable to make any changes to the document (including typing in the gray-shaded form fields). This only happens if you have Microsoft Word 2013, Word 2016, or Office 365. The message "this modification is not allowed because the selection is locked" or "this modification is not allowed because the document is opened for viewing only" may appear at the bottom-left corner of the Word window.
This problem is caused by Word opening the document in "reading view," which is a new way of viewing documents added in Word 2013. This view restricts you from editing the document (it is intended as a sort of e-reader type view).
Above: a document opened in reading view in Microsoft Word 2013.
To edit the document, you simply need to switch to the normal "print layout" view. You can do this by clicking the View tab at the top and then clicking Edit Document. (You can also click the "print layout" view button at the bottom-right corner of the window, next to the "zoom" control.)
To stop Word from always opening documents in reading view, click on the File menu and click Options. Then, under the heading Start up options, un-check the box labeled "Open e-mail attachments and other uneditable files in reading view," then click OK.