The Draken 3.2 Flight Manual is in Microsoft Compiled HTML Help format (chm). The source files were created five years ago in Robohelp, an application I do not have access to anymore. So there will probably never be a pdf version.
What do you mean by “cannot open”, do you get an error message or does nothing at all happen? Are you referring to the actual help file or the zip archive? What version of Windows are you using? You should provide more information if you expect to get some help.
But lets assume that you are using Windows 8. It is well known that the chm format is badly supported in Windows 8, and you can find solutions to this in many places on the Internet. Try googling “chm windows 8”.
Good luck /Tom
Win 10 Pro | P3D v4.5 | AMD Ryzen 3 3.5 GHz | 32 GB RAM | NVIDIA GT1030 | TM HOTAS Warthog | TM HOTAS Cougar