The format question only has one wrong default: assuming whatever the file already is in is the right thing to send. Almost every document benefits from a moment’s thought about the format before it goes out, and for most of them, the answer is the same.
PDF is the right default for almost everything
If the recipient is meant to read the document rather than edit it — a CV, an invoice, a signed contract, a report, meeting notes, a flyer — send a PDF, not the source file. A PDF locks in the layout so it looks the same on their screen as yours, opens on any device without needing the software that made it, can’t be accidentally edited or overwritten in transit, and reads as more finished and professional than a native file arriving in someone’s inbox.
- Word document → Word to PDF
- Spreadsheet → Excel to PDF
- Slide deck being shared to read, not present from → PowerPoint to PDF
Send the native file only when editing is the point
The exception is real: if you’re collaborating on a document — someone needs to add their section, adjust the numbers, or leave tracked changes — send the .docx or .xlsx. A PDF actively gets in the way here, since editing it requires converting it back first. The tell is simple: are you sending this so someone can change it, or so they can see it? Send native for the former, PDF for the latter.
Consider the recipient’s software, not just yours
A .docx built with a recent Word feature can render oddly — or refuse to open at all — in an older version of Word, a competing word processor, or a mobile app with partial format support. A PDF sidesteps this because the rendering was already decided and locked in before you hit send; nothing about the recipient’s software affects how it looks. This matters most for documents going to someone whose setup you don’t control: a client, an application portal, a stranger.
When even a PDF is more than the situation needs
If the destination is a place that doesn’t display documents at all — a social media post, a WhatsApp status, a webpage that only accepts image uploads — converting to an image is the actual right call, not a downgrade: see How to Post a PDF on Instagram, LinkedIn, or WhatsApp Status.
Size and sensitivity, before you hit send
- Over your email provider’s attachment limit (commonly 20–25 MB)? Compress the PDF rather than sending a link to cloud storage as a first resort — a link the recipient has to click and possibly request access to is friction a straightforward attachment avoids.
- Sensitive content — a payslip, an ID scan, a medical letter, a signed contract? Format choice alone doesn’t protect it. See How to Email Sensitive Documents Safely for encrypting the file and sending the password through a separate channel.
- A spreadsheet you don’t want edited or resorted, even by someone well-meaning? Share a Spreadsheet Nobody Can Change covers why PDF is usually the safer send there too.
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