If you already have the file downloaded you can convert it easily from EBCDIC to ASCII in a Linux or MacOS machine by using the command line. To accomplish that you need to use the dd command.

Here a quick overview of some parameters it uses:

dd [bs=size] [cbs=size] [conv=conversion] [count=n] [ibs=size] [if=file] [imsg=string] [iseek=n] [obs=s] [of=file] [omsg=string] [seek=n] [skip=n]

There are more parameters that those above, to check all available just do the command: man dd, it will show all other available parameters and the explanation of each one.

In your case you should start with:

dd conv=ascii if=EBCDIC_file.txt of=ASCII_file.txt

where EBCDIC_file.txt is the filename of your input EBCDIC file and ASCII_file.txt will be the file created as output with all bytes converted from EBCDIC to ASCII.

Likewise you can do the reverse by using conv=ebcdic to convert a file from ASCII to EBCDIC.

Here's the man page for dd on the web: https://www.man7.org/linux/man-pages/man1/dd.1.html

When you mention compressed in your file, do you mean the whole file comes compressed from the mainframe? Probably it came TERSED (by using terse utility on mainframe). If this is the case, there is a public version of terse that runs on DOS, Linux, MacOS, AIX and others. It is available on cbtape site: http://www.cbttape.org/ftp/cbt/CBT892.zip