CHM files are compressed in a binary format with the .CHM extension. The format is for software documentation. Microsoft introduced it as the successor to WinHelp with Windows 95 OSR 2.5 and Windows 98.
CHM stands for Compiled HTML. The format compresses and deploys files. If your CHM file has an eBook, open it in Calibre. Calibre is multiplatform. Or use another eReader supporting CHM.
To convert a CHM file, extract the .HTM files with G.D.G Software Chm To Exe. Also images and other files. You can also use Chm To Exe to convert to .EXE format. To complete to EXE, install G.D.G Software HTML Executable.
If your CHM file is problematic, ensure it is complete and error-free. If from a third party, ask them for another copy. The file may have copied wrongly. That stops you accessing it.
The LIT extension modifies the CHM eBook file format. Many extended characters are supported despite lacking Unicode. Compression uses LZX. Decompress with HTML Help Workshop and 7-Zip.
Programs opening CHM documents include: Beyond CHM, Microsoft Internet Explorer, Microsoft Reader.
Microsoft HTML Help
XCHM Viewer