In short
PDF — and only PDF — up to 100 MB. If your file is a .docx, an image or a spreadsheet, export it to PDF first: every office suite offers it, and so does the print dialog of every operating system.
Why a single format
A PDF locks the layout: margins, fonts and page order look the same on every device. Faxing relies on that, because each page is turned into an image and printed at the far end.
The ceiling is 100 MB per file. A password-locked or corrupted PDF cannot be opened by our servers, so remove the protection before attaching it.
Making a PDF out of anything
In Word, Excel, LibreOffice or Pages, go to File, then Export. In Google Docs, choose Download, then PDF Document. For everything else — a web page, an invoice on screen, a business application — open the print dialog and pick a virtual PDF printer.
Several files to send together? Merge them into a single PDF first, for instance in macOS Preview by dragging thumbnails across, or with any merge tool: one send carries one document.
Shrinking a heavy file
A huge PDF nearly always contains full-resolution photos. Fax works in black and white at roughly 200 dots per inch, so rescan at 200 to 300 dpi in greyscale, or compress the existing file. Nobody will see the difference on the recipient’s paper.
Key points
- One format: PDF.
- Maximum size: 100 MB per send.
- Export or “print to PDF” from any application.
- Remove passwords and merge files before sending.