Hi. I've been having a problem with PDF's in Illustrator. I often draw line work drawings in AutoCAD and print that to PDF. Then I import the PDF into Illustrator so I can create graphical diagrams. The problem I'm having is that when I place the PDF into Illustrator, the PDF appears really faded. I always have to go back to CAD just to darken all my line work. The PDF's themselves look perfectly fine when I open them, but once I place them in Illustrator it looks faint. I noticed that if I place a white box behind everything, covering the entire art board, it helps make the PDF show up better, although even when I do that, it's still not as dark as it should be.
I have been experiencing the same issue. Even when I change my line weights, line colours and line thicknessess my image on Illustrator experiences minimal to no change but the changes on the initial export pdf are drastic. I can no longer increase or change any of my lines without distorting my lineweights. The image on illustrator is, however, alomst not visible.