I currently have several Microsoft Word documents that are sitting on my Google Drive and were uploaded over the past few months. I understand that it is possible to convert Microsoft Word documents to Google Docs format during file upload time, but is it possible to convert documents after uploading them to Google Drive without having to download and re-upload the same document again?
You can use Google Drive add-ons such as:
Convert Word and Excel files to PDF and more.
Convert files from your browser without downloading any conversion software.
This app allows you to save Dropbox, OneDrive, Box, Flickr files to Google Drive directly.
Supports convert docx, pptx, xlsx, pdf, odt, jpg... formats to Google Document Formats.
For Google format to PDF, check: How do I convert a Google doc to PDF without downloading it?