Here is a how-to for converting a PDF to Markdown in order to copy-paste it into the Discourse Composer! There are lots of ways to do this. I think I tried them all (with mediocre results) before discovering this process.
- Open the PDF in Acrobat PRO (the free Acrobat Reader won’t do **)
- In the top menu select File → Export To → Microsoft Word
- Save the File to the Downloads folder on your computer
- Next, go to your Google Drive and click + New → File Upload
- In your computer’s Downloads folder, find that Word (.docx) file, and select it (or click the Save button to upload it to your Google Drive
- Once you see the uploaded (.docx) file listed in your Google Drive, double-click the file to open (it will open in Google Docs and be automatically converted to Google Docs format)
- Next, there in Google Docs, you need to enable markdown
- To do this, go to Tools → Preferences and check the box next to "Enable Markdown” (if it isn’t already checked)
- Select the content you want to copy. To select everything go to Edit → Select All
- Right-click on the selected content, and in the resulting popup menu, click on Copy as Markdown
- Now you have the markdown version of the content copied into your computer’s clipboard
- Next, go to edit your Discourse Topic or Post and paste in the copied content
- To do this, right-click in the text area, and in the resulting popup menu, click on Paste
- Save!
** If you don’t have Acrobat PRO you can use Adobe’s online converter here a limited number of times)
REFERENCES:
https://www.adobe.com/acrobat/hub/convert-pdfs-for-google-docs.html
Use Markdown in Google Docs, Slides, & Drawings - Google Docs Editors Help
(Hello to my future self! 
)