There are items/options that can be used within a given PDF that will
drastically affect how likely it is that the PDF will still be readable.

* Inclusion of 3D applets or any Adobe Acrobat specific features
	I have seen PDFs with 3D chemical applets embedded somehow into the PDF
using a plugin. The longevity of this addon depends entirely on how long
the company/team that made the plugin, wants to support it.

* DRM of any kind - password-protected, print-disabled, etc
	These features make it hard, verging on the legally impossible to
migrate the PDF to a newer format or read it on newer versions of
Acrobat or other PDF viewers. (Legally impossible, as circumventing this
would incur the wrath of the DMCA)

* Any other odd features.

There is a profile, which doesn't allow you to add any of the above, and
it is often referred to as PDF/A (A for Archival format)

The easiest way to create these at the moment, is to use OpenOffice 3
and choose the "Save As PDF" and tick the PDF/A option.

As for not becoming unreadable.. well, this all depends on age (and so
the version) of the PDF, and your current viewing software. I have
already had situations where older PDFs cannot be viewed correctly in
newer readers (majority of these were due to older 'print-ready' pdfs
with colour-information held within)

And this doesn't include the various issues that can arise from fonts
not being included or present on the client's system, 'print' fonts that
compress letters in interesting ways ("fi" -> single character, but a
non-unicode one), images that do not display, incomplete PDFs due to bad
exports that silently fail, etc.

My advice is to keep the source files alongside, especially if they are
in (la)tex or HTML. Text is always parsable.

Ben

On Mon, 2009-06-15 at 11:37 +0100, Mike Taylor wrote:
> Dear CODE4LIB colleagues,
> 
> In one of my alternative incarnations, I am a zoological taxonomist.
> One of the big issues for taxonomy right now is whether to accept as
> nomenclaturally valid papers that are published only in electronic
> form, i.e. not printed on paper by a publisher.
> 
> In a discussion of this matter, a colleague has claimed:
> 
> > [PDF files will not become unreadable] in the next 30-40 years.
> > Possibly not in the 20 years that will follow. After that, when only
> > 30-year and older documents are in the PDF format, the danger will
> > increase that this information will not be readable any more. It is
> > generally considered as quite unlikely that PDF will be readable in
> > 100 years.
> 
> I would appreciate any comments that anyone on this list has on the
> likelihood that PDF will be unreadable in 100 years.
> 
> Many thanks,
> 
>  _/|_	 ___________________________________________________________________
> /o ) \/  Mike Taylor    <[log in to unmask]>    http://www.miketaylor.org.uk
> )_v__/\  "Can't someone act COMPLETELY OUT OF CHARACTER without arousing
> 	 suspicion?" -- Bob the Angry Flower, www.angryflower.com