Broken PDF output from WEB and CWEB input

Dear all,

I'm in the process of creating the "library" of PDF documents for many 
of the CWEB and WEB sources in TeX Live with both pdfTeX 1.40.21 and 
XeTeX (xdvipdfmx 20200315).

The following issues occur with the set of PDF files in this tarball: 
https://github.com/ascherer/web/releases/download/2021-02-07-RC4/pdf.tar.gz 
(files "*-changes.pdf" were created with XeTeX and they work fine); see 
the single attached PDF 'vftovp.pdf' as MWE.

(1) For quite some time I have noticed that the output from pdfTeX does 
not work properly on Android 7.1.1 with MuPDF 1.11: More often than not, 
the pages are incompletely displayed, sometimes starting with an almost 
empty table-of-contents.

(2) Today, I tried to open 'vftovp.pdf' on MacOS 10.15.7 with Adobe 
Acrobat Reader DC 2021.001.20135.  This opens a popup window with an 
error message and if I don't close this (and Acrobat), after about half 
a minute the whole system freezes dead (hence no screenshot).

(3) If I stick the 'vftovp.pdf' file in the online PDF validator 
(https://www.pdf-online.com/osa/validate.aspx), I get a long error 
report (see attached screenshot).

I can, of course, create the C/WEB PDF library with XeTeX exclusively, 
but I'm curious why the otherwise trusted pdfTeX fails so miserably with 
the TeX input created from CWEAVE with 'cwebmac.tex' and WEAVE with 
'pwebmac.tex' (similar code from 'cwebmac.tex').

Regards,
Andreas