AC_ File

What are AC_ files and how to open them

Are you having problems opening an AC_ file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your AC_ files.

What is an AC_ file?

An .AC_ file is a CaseWare 2005 Compressed file.

A .AC_ file is a compressed archive created by CaseWare Working Papers. It packages an entire "client file" - that includes the main .ac file plus all its associated database files, documents, and optionally any backup (.BAK) files and subfolders - into a single file. The purpose is to save disk space and make copying or moving the client file easier when it's not being actively worked on.

CaseWare Working Papers software both creates and reads .AC_ files. Users can compress files manually or configure Working Papers to compress them automatically upon closing. To work on the contents, Working Papers must decompress (uncompress) the .AC_ file. A compressed file retains protection settings, so a login is required to decompress and open it if protection is enabled.

Files in this format begin with the standard Compound File Binary format signature (bytes D0 CF 11 E0 A1 B1 1A E1 at offset 0), include the ASCII string "Root Entry" around offset 1024, and contain the internal marker ".ACC5".

How to open AC_ files

Important: Different programs may use files with the AC_ file extension for different purposes, so unless you are sure which format your AC_ file is, you may need to try a few different programs.

While we have not verified the apps ourselves yet, our users have suggested two different AC_ openers which you will find listed below.

Last updated: August 16, 2026

Various apps that use files with this extension

These apps are known to open certain types of AC_ files. Remember, different programs may use AC_ files for different purposes, so you may need to try out a few of them to be able to open your specific file.

Help us help others

File.org helps thousands of users every day, and we would love to hear from you if you have additional information about AC_ file formats, example files, or compatible programs. Please use the links below or email us at submit@file.org to discuss further.