.adf files (the images of Amiga disks) are 880K. An amiga can only read a PC disk that has been formatted to 720k. As you can see, 800k will not fit on a 720k disk so the file will have to reduced in size (Zip, lha, RAR etc...)
Amigas don't use .adf files so this disk image has to be converted and written back to an amiga disk. Transdisk is the program to do this with.
While the process works, it is very time consuming. Also if you have anything less than 1.5 meg in your amiga then things become a bit complicated. Only got one drive? Then we are talking loads of disk swapping with less than 1 meg.
Due to the way a PC works, it cannot write to a disk that has been formatted as an amiga disk so all converting/decrunching has to take place on the Amiga which if you are talking about a 7MHz Amiga 500 will take AGES!