This article will the describe the process of opening and saving a PDF in Adobe Acrobat, if it is not currently your default PDF viewer. This is neccessary to ensure that fillable PDF's can be easily accessible across platforms (Windows, Mac, mobile devices).
Change your default PDF viewer to Adobe Acrobat (or Adobe Reader)
Right-click (ctrl+click) on the icon of any PDF. Select Get Info from the menu (pictured above).
Under 'Open with:' Select either Adobe Acrobat (or Adobe Reader). If you do not have Acrobat, you can download Adobe Reader, here.
Be sure to select "Change All..." in the above menu.
Making this change will ensure that all files of this type (.pdf) will open in Adobe Acrobat (or Adobe Reader).
This is an example (above) of how a pdf may look, if it is not set to open in Adobe Acrobat (or Reader) on Windows 10.
Right-click on the pdf icon, and drag your mouse over the "Open with" section. Select "Choose another app".
On this screen, select Adobe Acrobat DC and check the box that says "Always use this app to open .pdf files"
Mac Users should avoid Preview
By default, Macs are set to open PDF's (and images) with the built-in application, Preview. While Preview does a good job at displaying a more general .jpeg or .png file, Fillable PDF's become incorrectly formatted when opened and edited in the application.