So far the comments in another thread did not really help me in my case:

Recently I included some PDF figure files into Inkscape to add some text. When I export them, the resulting PDF misses some paths from the original file. What is the problem with exporting the SVG file, which is attached below, even as is?
I speak specifically about the path with in lines 53-56.

My Inkscape version is 0.92 and I see the issue in both Linux and Windows.

That blue swish kind of object is a pattern fill, inside a rectangular path, rather than being a path itself. It seems like I've recently heard that the pattern fills don't get converted properly to PDF (although I'm not sure if it's a bug, or whether it's been reported).

There are at least a couple of nested groups and clip paths involved, but here's how you can fix it without too much trouble.

Position your mouse over the blue object. Also locate the Style Indicator section of the status bar, which is the bottom-left corner of the window. Specifically the part where it shows the Fill and Stroke. Double-click on the blue object once, twice, it took me 3 or 4 times double-clicking, until you see where it says the Fill is a Pattern.

Next, click Object menu > Patterns > Pattern to Object

Now, I'm able to save the file as PDF, and the blue object appears as expected.