Visible vs. hidden
Using various tools, including Acrobat Pro, it is possible to add an invisible, behind-the-scenes summary to a table in a PDF. However, whilst a table summary is often a great asset, the behind-the-scenes variety is of limited value because it will be available only to NVDA (screen reader) users (and no one else). You will get far more bang for your buck if you include the summary text visibly on the page.
When a table summary is beneficial
If the answer a reader is seeking to extract from a data table is contained in just a single cell, as perhaps in the case of a train timetable, a summary may not be needed. However, if the reader is looking to identify any trends, patterns, relationships, proportions or anomalies in the data, then a well-constructed table summary can be highly beneficial.
How does it help?
Screen reader users
If you are a screen reader user and are looking for trends or patterns etc in a table’s data, in the absence of a summary, you will have to navigate through the table, cell by cell, to build a mental map of what is going on. (The equivalent for a sighted reader might be to have to read through the table, one cell at a time, whilst looking though a thin tube.) In all but the simplest of tables, doing so will require a great deal of time and effort. A well-crafted summary can do much of the heavy lifting work for you.
Screen magnifier users
Everyone else: the lessons of HTML5
Of course, there used to be a table summary attribute in HTML4/XHTML1.0 (the
<summary> tag) which did the same job of providing a behind-the-scenes summary. This tag was deprecated in HTML5 because table summaries are now widely considered to be potentially useful for all readers. Hence HTML5 document authors are encouraged to add table summaries on the page, visible to all.
How assistive technologies read behind-the-scenes PDF table summaries
Assuming a table and its caption have been tagged correctly, if you press “T” to go to the next table, NVDA will read out by default a behind-the-scenes summary, if one is present. Using the down arrow will then take you to the caption. (If there is no summary present, NVDA will read the caption by default.)
JAWS, ZoomText, SuperNova, Read&Write Gold and Claro Read
They don’t. NVDA is the only assistive technology (AT) that will read a behind-the-scenes summary. And, given the HTML5 position, it seems unlikely that AT support for hidden summaries will increase much, if at all.
Table summaries are useful for most types of table and for facilitating most types of readers’ queries. So, don’t hide them from view—place them directly on the page, preferably just before the table they describe.
For more on this topic, and a detailed study of the wider topic of making accessible PDF tables, please check out my ebook Design and build accessible PDF tables”. (It’s a few years old now, but still as relevant as ever.)