At the top of the InDesign forum page:
ANNOUNCEMENT:URGENT NOTIFICATION: Beware of fraudulent support requests.
We have received a number of reports where some members of this community are posing as Adobe Employees by offering to help troubleshoot issues personally using technologies such as Skype as a means to gain access to your computer and potentially your personal information. All Adobe employees are required to have a red ‘STAFF’ badge below their profile picture, as well as an Adobe logo after their username. You can also see both the 'STAFF' badge and Adobe logo if you mouse-over their profile picture.
When you make a statement like: "If someone is an expert I would be happy to have someone sign on to the screen and pay them for their time to fix it." you are just asking for a world of hurt. Scammers who troll these forums would like nothing better than what you are begging them to do. And you want to pay them?? ! !
There is probably a graphic (or possibly a font) in the document that it is keeping it from making a PDF successfully.
Use the "divide and conquer" method: Export a PDF of the first half. Then export the second half. Probably one of the two failed, but other was successful. For the half that failed, make a PDF of the first half then the second half. By subsequently dividing the file, you'll find the problem page. Then try removing the graphics one-by-one until you see the problem. The graphic may have to be resaved in its originating application.
Steve is spot on with the "divide and conquer" method.
However you can try something different, if you like:
1. You could use the "old" export method, that does not require a background task (CS4 and below). It requires a scripting functionality that is still available under the hood with the latest InDesign version. Several years ago I wrote a script, that is using this method and is still working with InDesign CC 2015.4.
More on that here:
2. In case you are exporting to PDF/X-1a, you could try to print to PostScript and distill that PostScript file through Adobe Distiller that comes with Acrobat Pro / DC to PDF/X-1a. Just to see, if that will work in your case. That method requires the use of the ADPDF9.PPD file that should be installed to make it available to InDesign here:
Applications/Adobe InDesign xxx/Presets/PPDs/ADPDF9.PPD
Create the folder PPDs if it is not there. Name it exactly like suggested.
You can download that PPD from my Dropbox account: