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I got a bit of a surprise when viewing a PDF today. I'd originally looked at it on my iPad in Safari. Then I looked at it later on my Linux machine using xpdf. It looked ... different. Here are the screenshots:

Safari on iPad:

PDF on Safari on iPad

xpdf on Linux:

PDF on xpdf on Linux

This is a document produced by Adobe, so I'd expect them to get the PDF stuff right. I therefore attribute it to the renderer rather than malformed PDF. Here's the specs for the PDF:

% pdfinfo /tmp/PDF_Transparency.pdf 
Title:          Transparency in PDF   Technical Note #5407
Subject:        Transparency in PDF files
Keywords:       PDF Transparency
Author:         Adobe Developer Technologies
Creator:        FrameMaker 6.0
Producer:       Acrobat Distiller 4.05 for Macintosh
CreationDate:   Fri Dec  1 16:41:40 2000
ModDate:        Fri Dec 15 16:53:01 2000
Tagged:         no
Pages:          82
Encrypted:      no
Page size:      612 x 792 pts (letter)
File size:      420449 bytes
Optimized:      yes
PDF version:    1.3

I got the document from the wayback machine at http://web.archive.org/web/20011109072226/http://partners.adobe.com/asn/developer/acrosdk/docs/PDF_Transparency.pdf.

As a mathematician, the appalling rendering of the formula is extremely worrisome.

I tested this in a couple of other PDF-capable apps and they had the same rendering as Safari with the - obvious! - exception of Adobe Reader which had the True Rendering. The fact that disparate apps give the same incorrect rendering makes me wonder if PDF rendering is something that is inbuilt to iOS5 and which most PDF apps use, but Adobe Reader chooses to ignore.

So my question(s) are:

  1. Is that assumption correct? If so, that makes reporting the bug easier as it implies that I can report it to a central place and if it gets fixed there it will propagate out. So if so, where do I report it?
  2. Are there PDF apps that get this right? I've listed Adobe Reader above as one that does, I can report that GoodReader, GoodNotes, iAnnotator, Explain Everything, iBooks do not. I've provided the source above to make it easy for people to check. The screenshots are from p7 (internal numbering, p13 absolute).
  3. Last one: was I just unlucky with this document? I've been happily using mathematically-heavy documents on the iPad without noticing anything like this. Should I be worried?
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For the record, the screenshot was taken on page 13 of the PDF. Chrome’s built-in PDF viewer renders it correctly: i.imgur.com/nFj4I.png And so does Preview.app on OS X Lion 10.7.3: i.imgur.com/7OHGg.png –  Mathias Bynens Mar 23 '12 at 8:37
Yes, sorry if that wasn't clear. The p13 absolute refers to the physical number of pages (as opposed to the page numbering within the document). Nice to see that OSX gets it right, but that doesn't help me much on the iPad! Unless there's a Chrome app for the iPad that I don't know about. –  Loop Space Mar 23 '12 at 8:44
This really isn't pertinent to me, but it's one of my favorites simply due to content. Great work! –  timothymh Mar 23 '12 at 11:16

2 Answers 2

up vote 3 down vote accepted

It seems to me that maybe not enough Fonts have been embedded in this ("old") PDF. I downloaded it on my Mac, opened it in Preview and exported it under a different name (without changing anything else).

GoodReader on my iPhone now shows the alpha´s correctly. Inspecting it with Acrobat Professional now shows more imbedded Fonts than before (and of course a newer PDF standard being used).

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I'll accept this one as it is the most useful for a user until the bug is addressed. Thanks for doing the detective work. –  Loop Space Mar 24 '12 at 19:26

The work around posted by Asmus works in Explain Everything as well.

Basically, Apple approved iOS apps use a built in PDF renderer in the iOS SDK. There are open source PDF renderer's that could be used within apps but they are incompatible with the Apple Store license.

The solution would be for Apple to address this in their PDF renderer and the fix would propagate to all apps that use it.

We can submit the bug to Apple via our developer account and hope that enough other devs raise the question for it to eventually get resolved.

Reshan Richards Co-Creator of Explain Everything

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That confirms my suspicion. That would be great if you'd report it. Thanks for stopping by to answer. –  Loop Space Mar 24 '12 at 19:25

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