Those are likely webpage snapshots, rather than PDFs, as indicated by the standalone javascript files.

OK Seems it may have been from a website serving malware javascript in PDFs downloaded. Here are the results, once scrubbed I no longer had the condition. Attaching them so others may be advised to be

that let me tell Zotero to strip all of the javascript out of all the saved pages (ideally with the ability to make individual exceptions, but useful interactive content is quite rare).

I'm not sure what you're referring to, but Firefox still has a "Work Offline" option. Which brings in a related point; some webpages don't work without javascript.

https://stackoverflow.com/questions/6291916/disabling-loading-specific-javascript-files-with-firefox

JavaScript Error: "ieeexplore-ieee-org.ezproxy.engglib.upd.edu.ph:443 uses an invalid security certificate.

However, I assume the sort by author of csl is implemented by JavaScript?

[JavaScript Error: "HTTP GET https://scholar.googleusercontent.com/scholar.bib?q=info:XQ7HOROs0bkJ:scholar.google.com/&output=citation&scisig=AAGBfm0AAAAAW5lsj4V8dhbkqe1D6j_RoKETfnDMrgL5&scisf=4&ct=ci

[JavaScript Error: "Cannot access contents of url "chrome-search://most-visited/single.html?title=Most%20visited&removeTooltip=Don%27t%20show%20on%20this%20page". Extension manifest must request permi

[JavaScript Error: "Could not establish connection. Receiving end does not exist.

[JavaScript Error: "HTTP request to https://ieeexplore-ieee-org-s-docadis-ups-tlse-fr.s.docadis.ups-tlse.fr/xpls/abs_all.jsp?arnumber=6697669 rejected with status 0"]

[JavaScript Error: "Error: Only 0 translators present in cache. Resetting

More importantly, Execute JS doesn’t work in the standalone Zotero app. Currently, the only access to the JavaScript API (like Execute JS used) is PyZotero.

[JavaScript Error: "TypeError: x.fileURL.endsWith is not a function" {file: "chrome://zotero/content/import/mendeley/mendeleyImport.js" line: 114}]

[JavaScript Error: "Translator Crossref.js with same ID is already loaded -- deleting CrossRef.js" {file: "file:///home/user/Zotero/translators/CrossRef.js" line: 0}]

@zuphilip all the professional adobe apps have javascript scripting enabled, yes, but I'm not sure if it is possible to call that from a different app, i.e. I know you can write a javascript automatio

[JavaScript Error: "Error for operation 'open' for /home/fattabby/Zotero/storage/24MCNUNZ/The Nature of the Firm.pdf: Unix error 13 during operation open on file /home/fattabby/Zotero/storage/24MCNUNZ

[JavaScript Error: "Cannot access contents of url "chrome-search://most-visited/single.html?title=Most%20visited&removeTooltip=Don%27t%20show%20on%20this%20page". Extension manifest must request permi

just in case that multithreading in javascript is not known, have a look here:

[JavaScript Error: "Translators not yet loaded"]