Help please! Need to convert .pdf into .epub

Aside from the evident confusion of Adobe apps here — and I am assuming someone said InDesign/ID could do this conversion — there are some problems with the process itself.

Other than that you can, I suppose, convert any file format into any other in the tech world (JPEG to text? Sure!), there is no good way to convert a PDF to EPUB format. The closest option I can think of is that some of the Amazon/KDP tools will accept a PDF for conversion to a page-image Kindle, but even the KDP mavens think it's a lousy option and result. I suppose amid the various EPUB file-munging tools out there, one might do some similar conversion (more likely, the other way around, EPUB to PDF) but again it would be a sort of worst-of-worst options to create a fixed-page, FXL EPUB file with limited conversion options.

I am not sure what "so [it can be used] on other platforms" means. There is a PDF reader, probably several, for just about every OS and platform that has a screen. PDF is by far a superior format for fixed, page-image documents; other than a few glitches with advanced features across nonstandard readers, and no good 'liquid' option, it's pretty much close to a perfect digital document solution. The only reason to convert it to something like EPUB would be to enable the document's controlled (DRM/protected) sale on platforms that don't handle PDF (which does lack any strong encryption or protection methods).

But in general, converting PDF to EPUB to enable its sale is about like crushing your car into a cube to make it easier to sell on the scrap metal market.

(ETA 1: To add a strictly technical answer: You can place a PDF in an InDesign document and then export to FXL EPUB, yes. (InDesign does not directly open PDFs, although a beta feature for that is in development.) But the result of this process would be at the low end of the poor alternatives for this conversion.)

ETA 2: If the original source document is available in any current/live format (Word, InDesign, most clones and workalikes, etc.) you'd have a better starting point for an EPUB version, hopefully in reflowable format and not fixed-page.

What do you mean by AdobeID?