Video Subtitles Specification [CHK and PAC]
This might be helpfull Subtitle-Edit for .PAC subtitles.
The .PAC subtitle format is binary format
For other subtitle format, .CHK is far I know the file format for "Chunks" or fragments of hard disk after doing CHCKDISK command. But also the CHK file extension is associated with the SSConvert application used to distribute data format containing multiple streams (typ. languages) of subtitle text, timing and presentation style data for video.