The tj operator is responsible for drawing text content. When such operator is left empty, the import throws NullReferenceException because of the missing operands. Available in LIB Version 2018.3.1022.
When there is long text that is out of the used cell range area, this text should be included in page splitting calculations.
Also, when a cell has text that overflows its width, the text is again clipped by the cell boundaries and is not visible in the exported PDF. In a similar case Excel exports the surrounding cells to ensure that the content is visible.
Workaround: Extend the used cell range to include all the cells which the text content spans.
German umlauts (the characters 'Ä', 'Ö', 'Ü', 'ä', 'ö' and 'ü') will not be printed to a generated pdf file, but the other German "special" character "ß" works.
I use the trial version via the Telerik NuGet feed. A test program and my locally generated file are attached.