'there was a problem reading this document (14)' and 'expect a dict object'

@Hannah2323 shared the PDF in question in a private message.

The issue:

Usually the trailer of a PDF has a Root entry which points to the Catalog dictionary which in turn points to several central document structures, in particular its Pages entry points to the page tree of the document.

In the file in question, though, the Root entry in its trailer points to an annotation dictionary!

Thus, while trying to open the file Adobe Acrobat cannot find the page tree in the alleged catalog object and rejects the file: Without the page tree there is no content in the PDF at all.

A work-around:

To somewhat fix this issue, I searched the page tree base object, added a new indirect dictionary object to the PDF with a Pages entry pointing there, and changed the trailer to point to this new indirect object as Root. Eventually I added an xref table entry for this new object.

This can be done in a text editor (as long as that text editor does not change regions of the file one doesn't work in) but requires some knowledge of PDF internals.

Thereafter the file could be opened and was mostly ok. Saving from Acrobat then finished the work-around.

A possible cause:

Inspecting the file a bit further one finds a linearization dictionary and two cross reference streams that look like they may have been the two cross reference streams of a linearized file. Furthermore, the object number of the original root entry equals the lower of the Size entries of the cross reference streams, i.e. of the stream that originally would have been located at the end of the linearized file.

Thus, it may be that the tool for the Lenovo pen does not support linearized files with cross reference streams and only considered that cross reference stream at the end, thinking that object number was unused yet. 

But this may also be a red herring, maybe the file that tool edited was already somehow broken and this is a situation of GIGO, garbage in, garbage out...