in general lines and rectangles are fixed objects that you can place on your form, but there they are then. Size fixed, position relatively fixed (depending on where you place them - in a flowed subform it might move with the previous content).
If the line is part of the document and for example must be placed at a specific position (e.g. Masterpage) or if you have a signature box that really is just an empty rectangle at a position in the document flow, then you can use the line/rectangle objects.
The strategy changes when you want to surround areas on the document that contain other elements or are variable in size. Also, when you place field elements and you want to draw a box around the entry part, using lines is awkward and unnecessarily complex. This is where you would use the border property of the elements or the surrounding subform(s).
If you cannot see the border of a subform this may have the following reasons (that I can think of the top of my hat):
- the color is white (happened to me once accidentally - not very likely though 🙂 ) - the subform is invisible or hidden - another element higher in the rendering layers overlaps and "hides" your subform (also not very common) - some scripting turns off borders - in a flowed subform with borders, when there is no visible content to resize the subform you may only see a horizontal line - you have set the visibility to "print only " and are looking at it in an interactive PDF (which is not considered print)
As @Mayank_Gandhi says - let us know what version of Designer and PDF you are using.