Hi,
I’m afraid it is due to the fact that is a .ppt document and not .pptx. The .ppt versions are very hard to read and convert, as those are binary documents. The newer, .pptx is a bit better, as it is a packed zip archive, and can be better parsed via PHP.
If you can, try converting them to .pptx files, that will very likely resolve the issue. Aside I am not sure if I can do anything about it, there is probably PPT file parser for PHP existing yet.