When opening a PDF in preview there often are streaks like this:
Does anybody have an idea where this is coming from?
There's a bug somewhere (maybe a PostScript bug related to Preview) in the Mac workflow when you Save as PDF. There is a rare occurrence where a line like that will occur, and the workaround is to repeat the Save as PDF command. The bug is rare, so you probably won't get a repeat the second time.
I've encountered the bug a handful of times because I frequently use Preview to compile screenshots (from online seminars) into a single PDF for later review. Here is the workflow I'm using when I encounter the bug:
Select all your screenshots, right-click, open with Preview.
Review the screenshots; right-click and delete any you don't want.
On the left, use [Cmd+a] to select all the screenshots.
Use [Cmd+p] to open the Print dialog, but at the bottom-left, choose Save as PDF.
After step (4), I find it necessary to quickly skim through the resulting PDF, just to confirm that a random line hasn't occurred. If it has, then I repeat step (4) and check again.
If my descriptions above are not helpful to you, then another thing to check is the software that was used to generate the PDF. For example, if you open a PDF in Acrobat (I'm not certain about Adobe Reader), then use [Cmd+d] to open the Document Properties dialog, and the Description tab will list the software used to produce that PDF. Knowing this, you can potentially identify the source of issues.