Uploading .pdf error(s)

Hey all.

Yes I know this has been asked/sort of answered in other posts but I have not been able to fix the problem with any of the suggested answers.

WP 4.7.2 Networked
Ubuntu
Child theme of Enigma
Custom theme
2016 theme

The sites have been working fine but today have developed a problem with some .pdfs, not all just some.

With some uploads I get the ever so helpful Http error.
With others I get Sorry, this file type is not permitted for security reasons – same file type though.
With others they upload fine, no errors, same file type.

I have changed the theme to default 2014, 2015 and 2016. Disabled all plugins.
Still get the same errors.

So it’s the files themselves. Eh? Same file types, created by the same program, not exceeding the upload limit (well beneath, only 1 – 2Mb in size).

Added ALLOW_UNFILTERED_UPLOADS to wp-config – made no difference whatsoever.

Images etc. seem to be uploading fine without any problems.

In the Apache error log I get a segmentation fault when I get the Http Error.

Wrote an upload script, tested with the same files and they upload without
any errors at all.

Stumped now.

Any help?

Cheers.

Ken.