In an ideal world, everybody on your projects would all be using the same version of AutoCAD LT or related AutoCAD-family products. Then you’d never have to worry about file format compatibility.
In the real world, though, you early-adopters sometimes have to help others out. That can mean saving your drawings back to an older format so their older CAD program can read it.
If you just have to do this once, change the file type in the Save As dialog:
You can choose anything from the current format (2010) all the way back to an R12 DXF. And remember, if somebody asks for a “2005 file” or a “2009 file” just use the next smallest number, okay? (2004 for “2005” and 2007 for “2009”.)
But if this is something you have to do all the time, you can make the change permanent in the Options dialog,
The drop-down contains all the formats you’d see in the Save As dialog, and below that are two more options.
“Maintain visual fidelity” is applicable to drawings using annotative scaling. If the box is checked, and you choose a file format of 2007 or earlier, then a new layer is created in the drawing for each scale, and the objects are separated out and put on those layers. (So you might end up with HATCH-48, HATCH-96, TEXT-48, TEXT-96, etc.) These layers are invisible as long as you keep opening the drawing in AutoCAD LT 2008 or higher. Checking this box does increase the file size, so if you’re worried about that and you don’t save drawings back too often, you might want to turn it off.
“Drawing size compatibility” is more about the size of individual objects in a drawing than the size of the drawing itself. Before AutoCAD LT 2007, objects were limited to an uncompressed size of 256MB. You *might* have a text or table object that would be that big, but it’s more applicable to 3D drawings with lots of meshes and surfaces. Overall drawings have a limit too, but it’s something like 4GB, so you probably won’t run into it either.
The short version is that if you’re regularly saving drawings back, you will probably want to leave these boxes checked, but if you’re always saving in the current format, unchecking them probably won’t hurt.