TL;DR

I have a very large Google Docs document which can not be exported to PDF*, but I can download it in all the other supported formats, which unfortunately do not fit my needs.

What are the limitations of downloading a Google Docs document as a PDF, why can't I export it into this format? (But why can I download it as a huge, 500 MB sized RTF?)

I didn't find any relevant stuff in the official documentations.

*: all the 4 of us who have access to this document tried with different Google accounts, same results.

In detail

I have a huge Google Docs document (shared with a team) containing 168 pages, multiple images, lots of equations, drawings, tables, etc.
When I want to download the whole document as a PDF _{(clicking "Download as" > "PDF Document (.pdf)")}, the browser _{(doesn't matter which one of the popular browsers)} starts to show the loading icon in the tab header (meaning it is processing the request), but about a minute later, it returns an "500 OK" HTTP Status Code _{(BTW not the usual "500 Internal Server Error", "500 OK" instead)}, which means it could not export the document to PDF.

Why? I don't think the reason is a size limit, because I can download the whole document in ALL the other currently supported formats such as .docx, .odt, .rtf, .txt, .html, and for example the downloaded .rtf file is 519 MB. _{(Really! The .docx format from the same document is only 71.8 MB, and the .odt is 48.7 MB.)} BUT when I cut this document down to about 50 pages, I CAN download it as PDF!

What are the limitations for downloading a Google Docs document as PDF?
Why can't I export this document to PDF?

Problematic workarounds

There are some workarounds, although unfortunately they produce other problems:

• if I split this document into at least 3 parts as separate documents, I CAN download the whole thing as PDF
  • it is obvious that it is a very uncomfortable solution (we want to edit the document as ONE large doc.), and this way all three documents should have separate table of contents part
• downloading the document in .odt (OpenDocument Text) or HTML format:
  • this is NOT a good solution in my case, because the equations get downloaded as images, and they get messed up if they contain special characters which are NOT part of the equation toolbar (like bowtie symbol ⋈ (U+22C8) for natural joins in relational algebra) or accented characters (á, é, í, etc.), and get substituted by multiple question marks (?). Here's an example:
    • equation in Google Docs (correct):
      
    • equation in .odt or HTML format (incorrect _{and uglier}):
    • equation in Google Docs:
      
    • equation in .odt or HTML format (incorrect _{and uglier}):
  • BTW this is exactly how these equations look like when I view the document from a tablet or a mobile phone (buggy appearance)
• downloading the document in .docx (MS Office) format:
  • in some cases, indentations and spacings get messed up and font types get changed for some reason
  • but without a doubt, this produces the most acceptable result

Here's the output of the browser's developer console when such a request is sent:

Sorry for being so verbose, I wanted to share all the information I could find out.