I have a non-standard opening where I need to install an interior door, and want to avoid having a custom door made. The closest off-the-shelf door that I can find at my box store would need about 1 inch shaved off the top and 1 inch off the bottom to fit the opening. (The horizontal width wouldn't need to be changed.) Can I take that much off an MDF door?
The answer depends on whether it is hollow or solid. If it is hollow then you need to find out how much is solid on each end. If it is solid then there is no issue. For example, taking one door somewhat randomly from Home Depot based on Interior Door + Hollow Core + not pre-hung:
This one is hardwood rather than MDF, but that really shouldn't matter much. What matters is the specifications from the manufacturer for the specific door. In this case:
Door trimmable for custom fit up to 1/4in T, 3/4in B & 1/8in S
If it is solid then you can trim as much as you want, but you pay a lot more for the door.
Probably not. You don't specify a manufacturer, but I found: https://residential.masonite.com/learn/doors-101/faqs/ and http://www.trustile.com/sites/trustile.com/files/attachments/2014-06-20_mdf_handling.pdf both of which say maximum 1" bottom trim, and no top trim for their MDF doors.
If it's poorly enough made, and many of these doors are, you can pry the covering panel away from the frame for a few inches, shorten the frame, and then glue and clamp it back together.
Better yet though, if there is a used construction material store near you, look for an older door. The older moulded doors look just like the new ones, but they are a lot heavier and can easily be trimmed to size.