"Z-order"
When manipulation occurs from the Order panel or Content panel PDF some page content objects can end up behind others.
PDF page content - text included - is a "painted" graphic object. Yes, I know, a head scratcher but it is all explained in delightful detail by ISO 32000.
To avoid this work from the Tags panel. Ensure that the PDF page content's logical hierarchy is properly developed in the structure tree.
AT uses the structure tree to determine what is what which includes order/sequence of semantic content on a PDF page.
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For those who need to or desire to provide accessible PDF the release of the ISO standard for PDF/UA is a "must have" reference.
--| The ISO Standard for PDF/UA (ISO 14289-1:2008)
Purchase at: http://webstore.ansi.org/RecordDetail.aspx?sku=BS+ISO+14289-1%3a2012
Purchase at ISO: http://www.iso.org/iso/catalogue_detail.htm?csnumber=54564
Three additional documents also belong on one's 'bookshelf".
--| Achieving WCAG 2.0 with PDF/UA (Available)
http://www.aiim.org/Research-and-Publications/standards/committees/PDF UA/WCAG20-Mapping
--| PDF/UA-1 Technical Implementation Guide
PDF/UA Technical Implementation Guide will be displayed when available at PDFUA Technical Implementation Guide
http://www.aiim.org/Research-and-Publications/standards/committees/PDF UA/Technical-Implementation-Guide
-| PDF/UA-1 Technical Implementation Guide: 32000-1
PDF/UA Technical Implementation Guide for ISO 32000-1 will be displayed when available to PDF/UA Technical Implementation Guide:32000-1
http://www.aiim.org/Research-and-Publications/standards/committees/PDF UA/Technical-Implementation-Guide-32000-1
Be well...
