With Microsoft WORD (2016) & PDFMaker:

With an Image with Transparency: .PNG File. Placed against a dark background (paper colour) to show the white outline that occurs around the image.

Occurs in .PDF files when No image resolution downsampling is set, and image types are stored in NON-lossy image formats, e.g. NONE, ZIP, JPEG 2000 (LossLESS)

It occurs at all zoom levels. But disappears when zoomed in to the maximum zoom level.

When the resultant PDF File is later, either DownSampled or LOSSY Image comprressed (Using PDF Optimiser), the white Outlines become part of the Image and are much more Visible.

Attached are files that show this bug: PDF, DOC, PNG, PDF (DownSampled & LOSSY compressed)