Hi 
Annoyingly PDF is another non-standards based file-format.
In much the same way as RTF, DocX it does keep changing at the whim of a
single profit-making company - usually this keeps their own product ahead
of all competitors products in terms of writing and also in reading that
format. With PDF it's Adobe's whims that keep changing the format and
constantly making it different from other implementations, including their
own.
So people are kinda forced into using Adobe's PDF reader and need to keep
updating or upgrading it - or just accept that whichever pdf-reader they
currently use may not handle all Pdfs. When people use Adobe's newer
PDF-writer the resulting Pdfs may be unreadable (or have large chunks that
are unreadable/usable) in anything other than the latest Adobe Pdf-reader
Non-Adobe PDF-writers continue to write versions of Pdf that can be read by
almost any Pdf-reader.
Use of Pdf has risen 'recently' due to an increasing lack of confidence in
documents created in MS Word. People are finding that even old documents
may not be read perfectly in newer versions of MS Word than they were
written with, or even the 'same' version on a different machine.
The Pdf format generally removes the ability to edit or modify documents so
any documents kept only in that format will need to be completely
re-written for even fairly minor edits to bring them up-to-date. Some
very minor edits may be possible thanks to programs such as LibreOffice but
such programs rarely allow text to re-flow nicely in the way people usually
expect of word-processors such as Writer, Word, etc.
Regards from
Tom
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