1. Which version of Word you are using?
   
2. Are there any pictures in your document, on the same page as where the text boxes are becoming invisible?

From my notes on this subject, I have the following:

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I was finally able to "induce" the problem by increasing the default resolution of the PDF reDirect printer from 600 dpi to 1,200 dpi or higher.

• When increasing the resolution, the conversion takes a lot longer as the memory needed is squared (600 dpi x 600 dpi = 360,000 dpi squared while 1,200 x 1,200 =  1,440,000 dpi2 >> 4x more memory).
• At the higher setting, Word will convert all of the pictures to a higher resolution equivalent. No details are gained, so there is no benefit unless your original resolution of your picture is also at the same higher resolution.
• It looks like either Word or the printer driver runs out of memory, and corrupts the construction of the file
• There is no advantage to a higher resolution unless you are going to a offset print facility with your PDF. Generally speaking, the human eye will have difficulty seeing anything higher than 300 dpi without a magnifying glass.
• The problem with PDF files is that it is so easy to "zoom in" and see the imperfection that you would not see on paper otherwise.

I was able to induce the problem only with Word 2007 and Word 2010. I tested Win XP, Vista and Windows 7. Word 2003 and Word 2013 (Win 8.1) do not have this problem, even if I set the resolution to 4,000 dpi. Therefore, I believe that the problem rests with Microsoft.

I hope this helps.

Michel