Why does drop-shadows gets distorted when inserted into a pdf?

After finding out how to extract images from a PDF and create new bitmaps I then tried to reinsert an edited image into a flight manual pdf.

I placed an image and corresponding drop shadow (same dimensions, two bitmaps) inside a pdf using "Replace Image" action in both "ApowerPDF" and "PDF-XChange Editor" for the same result. Shadows in lower right corner are longer than shadows in the upper left corner. As if the shadow-drop map has been stretched and have different dimensions from the image.

What is happening? Merging the bit maps in GIMP looks fine.

Image merged in GIMP. Drop shadows are fine (equal all page).

Same two bitmaps inserted into Flight Manual. Drop shadows elongates towards lower right corner.

PDF forces size and stretches I think.

Trial and error. Create two bitmaps, one with black X's in each corner and one with red X's in each corner.

Assuming red X's is on the 'inside' bitmap (wholly covered by the other bitmap). Place bitmap with black X on the pdf page (using apowerpdf). Adjust:

• red X bitmap layer offset X, Y from upper left corner
  
• red X bitmap canvas size
  

until red and black X's line up. Then offset and size is now known to make the shadow-drop line up.