PDF page content export reflects the "under-the-hood" quality of how the PDF was made.
This of course is driven, in part, by what the authoring application was, how content was mastered and what process was used to actually paint content to the PDF page.
http://talkingpdf.org/each-pdf-page-is-a-painting/
So --- GIGO is operative.
Goodstuff In = Goodstuff Out
OR
Garbage In = Garbage Out.
The export results tell one which of the above is in play.
As well, 'good' export of PDF actually has a prerequisite.
That is one needs a well-formed Tagged PDF.
Tagged PDF has two main design criteria -
Support Accessible PDF.
Support export of PDF page content
(See ISO 32000)
Having an ISO 14289 (PDF/UA) compliant PDF provides the "well-formed Tagged PDF".
Aside from ISO 3200 and ISO 14289 additional useful information is available at AIIM's web site.
http://www.aiim.org/Research-and-Publications/Standards/Committees/PDFUA
Be well...