I generated a PDF from a document that contains many images, mostly JPGs, but some PNGs.
In the doc (Libre Office .odt), the page in question looks like this:
When I "exported" the document to a PDF, that page appears like this:
Flummoxed, I copied the image from the .odt file and pasted it into Paint. I then selected File > Save to see what image type it defaulted to. It was PNG. So, on a hunch, I saved it as a JPG. I replaced the PNG file in the .odt file with the new JPG. I generated a new PDF. The image now generates correctly.
Was this a fluke, or are JPGs more reliable/the preferred image type for use in documents that will be PDFerized?
I replaced six images that were goofed up like this; they were all PNGs until I saved them as JPGs and then replaced the original PNGs with JPGs. On re-generating (exporting) as PDf, all problems were solved.
I did notice too, though, that at least one of the images was quite large (13 MB). I used Irfanview to "halve" it, which took it down to 2.3 MB; I then halved that image, which brought it down to about 690KB. It still looked fine at the size I needed.