Sending a document to read is different from sending one to edit, sign or print. Pick the app you wrote it in and what they need to do. We'll say which format to send, what they'll lose, and the converter that gets you there.
A PDF records where things appear on the page, not what they are. Converting back rebuilds the document from those positions, so some features come back as imitations. Pick a feature to follow it through each step.
DOCX, XLSX and PPTX, along with ODT, ODS, ODP and EPUB, are ZIP archives of XML files. Drop one in and your browser unzips it, then reads the parts that travel with every copy you send: authors, editing time, deleted wording kept as tracked changes, comments, hidden text, hidden sheets and slides, speaker notes, macros and external links. Everything happens on your device. Or open the sample, a tenancy agreement built in your browser just now.
Count it, compare versions, fix the capitals, or write it in Markdown. Each runs in the browser as you type.
RFC 4180 only defines the commas, quotes and line breaks. It says nothing about character encoding, dates, leading zeros or which character separates the columns. Edit the file below. The left grid is a strict RFC 4180 parse. The right grid shows what a spreadsheet typically does when you double-click the file. The encoding panel shows the actual bytes.
The spreadsheet column models Excel's default behaviour when it opens a .csv by double-click. It isn't Excel itself. Recent versions let you turn some of these conversions off, and the Data → From Text/CSV import avoids most of them. Opening the file with CSV to XLSX sets each column's type explicitly.
Each page explains what changes on the way, with a live demo of the tricky parts. The family converters handle the less common pairs.
Most modern office files are ZIP archives and start with the same four bytes, 50 4B 03 04. What tells them apart is a part inside: [Content_Types].xml for Microsoft, or a mimetype file for OpenDocument and EPUB. The older binary formats share a different signature, the OLE2 compound file header.
The other toolboxes, with a typical job for each.