A Writer document may have a background image as part of the page style. Such an
image can appear as a "watermark" if the image is mostly transparent. When text
frames, which are otherwise invisible, overlap colored areas of the background
image, they produce visible rendering artifacts in an exported PDF.

Steps to reproduce:
1) Open the sample document
2) Export to PDF
3) View the PDF in Adobe Reader

Notes:
* The sample document has a simple black PNG image (75% transparent) as the
  background. This is just for demonstration, to make the problem easier to see.
* There is a rendering problem in the Writer Page preview as well: the 50%
  white frame (middle bottom row) does not appear initially, but only after the
  area is covered by another window and then re-exposed.
* The line-artifacts are much more difficult to see using the free evince PDF
  reader, but they are still present. I haven't tested any other PDF readers.

Workarounds:
* Instead of a transparent background image, use an opaque image. In an 
  image-editor, create a white background layer under the transparent image,
  then 'flatten' and export to an opaque image to be used as the Writer background.
* Or, use frames that have a color-fill (e.g. white), at 100% transparency