August 21, 2014 4

Screen shot demonstrating an example of the z-order problem in tagged PDF.PLUS6 ways to beat the z-order problem in the first place!

It’s the content reordering nightmare. You are manually tagging a PDF in Adobe Acrobat. All of a sudden stuff starts disappearing from the page.

Professionals call it the “z-order problem”, but it’s known to most users simply by the howl of pain it so often provokes.

Many ask “is there a way to avoid it”.  There are a few key mitigation strategies that can really help reduce the need to go to Adobe Acrobat’s dreaded Order panel.

Why does it happen?

What’s going on is that Acrobat assumes you want to group all the objects you select – of any type – into a single tag. It’s not smart enough to say (if applications could talk): “Hey, my user selected a big block of text that looks like a paragraph, and he’s going for a <P> tag. Maybe I should ignore the fact that he’s also selected a large image with that paragraph!”

The current-generation Acrobat Professional XI software isn’t great at this sort of question. Accordingly, to minimize the z-order problem when manually tagging with Acrobat it’s important to adopt three key strategies. Other software may have similar features – check your documentation!

Select what you want. Unselect what you don’t

The only way to conclusively avoid the z-order problem when tagging by hand using Adobe Acrobat is to be sure that you select exactly and only the items you want tagged. This isn’t always easy. You must avoid non-text objects when you want to add text tags such as <P> or <H2>. Likewise, you must exclude unassociated objects when you select content (text or graphics) that you want grouped in a single tag such as a <Figure>.

Let’s get to the tips!

Tip 1. Get familiar with Acrobat’s selection tools

Tags panel context menu showing "Highlight content" selected.In order to pay careful attention to your selections you need to be able to see what you are doing! In the Tags panel, be sure that “Highlight Content” is selected. You’ll then be able to rely on the faint blue boxes that surround objects you’ve selected. Verifying this setting is the very first thing I do before I even think of touching a tag. Frankly, I don’t even know they make it possible to turn this feature off.

If you are using the Touch Up Reading Order Tool you’ll get the selection highlights regardless of the Highlight Content setting. Due to its crudeness, however, I don’t encourage the use of this tool, but that’s a subject for another post.

Method 1: shift-click to add content to the selection

Screen shot showing mouse cursors for adding and subtracting content from a selection.

After using the mouse to “zone” a block of content, use shift-click to add content to your current selection.

Note that the mouse gets a plus sign to indicate you are in “add content to selection” mode.

Method 2: control-click to remove content from the selection

Alt-click (Windows) or option-click (Mac OS) to zone content for removal from the currently selected content.

Note that the mouse gets a minus sign to indicate you are in “remove content from the selection” mode.

Tip 2. Save your work at 5 or 10 minute intervals

You can’t really undo from content-reordering. Accordingly, be sure to save your work at very frequent intervals – hopefully, right before you make a move that screws up the z-order. Often, ordering is screwed up in several places, or putting it back into the right order is daunting for some other reason – usually because there are lots of objects on the page.

That’s why I prefer not to try “fixing” z-order problems when they occur. It’s usually easier and safer to go back to a PDF from just before you made the move which screwed up the page.

Save yourself the suffering, and dump that file – revert to your last save instead!

Tip 3. Review the whole PDF before starting manual work

If you are starting from an untagged PDF, be careful! Acrobat’s “Add Tags” feature is capable, all by itself, of screwing up the z-order even before you’ve started with manual tools.

Flip through the PDF before and after running “Add Tags”, or use Acrobat’s Compare Documents feature. If you see z-order problems before you’ve started manual tagging, stop and think through how you are going to address the problem.

  • If the problem is restricted to only one or two pieces of content, maybe you can fix it in the content panel, but that’s a last resort, especially with complex pages
  • If significant numbers of pages are damaged you have a more serious problem. You may need to recreate the PDF, rasterize some content, or otherwise change the affected pages so they survive the Add Tags function in better shape.

Want more? 6 ways to avoid z-order errors in the first place

  1. I’ll go ahead and say it now: Quark XPress and Microsoft Publisher make PDFs that are really hard to tag. Avoid these applications when tagged PDF is the objective
  2. Put graphics you don’t want tagged into the page’s “background”. For example, in PowerPoint, put images that should be artifacts (and thus won’t get Alt text) into a template
  3. Avoid creating overlapping objects. Even if the content appears visually distinct the objects themselves may still physically intersect. Be especially careful of graphics that span significant portions of the page
  4. If your images or diagrams are comprised of smaller objects, group them in your source application, or if necessary, convert them into a single image. This simplifies the tagging problem for Acrobat – and for you, if you have to do manual cleanup
  5. Avoid placing one object on top of another to create a specific visual effect (a practice known as “masking”)
  6. Use PDF creation software that creates tagged PDF rather than adding tags following creation