THE PRINTER WIPES THE TRAIL. Online journalists often attach an unpublished document to the story they are writing. Readers who only want the gist will stick to the article. Those who want to dig deeper can open the full document, linked from the piece. This article-plus-document approach is sound. It makes journalism more complete. But it comes with a risk. Imagine the document is confidential, and a source has taken the responsibility of sending it to us electronically. That confidential file — say a PDF — may carry traces that could expose the source’s identity. Those traces, in the form of metadata, are easy to find. You can uncover them, for instance, by opening the document in a widely used program such as Adobe Acrobat Pro, right-clicking, and following this path: File > Document Information > General. To stop someone poking around in a document we have just published — with the aim of unmasking our source — we should use a protection strategy, especially when the source is valuable. Open the document and print it on paper. Then go to an office printer with two advanced functions: it can scan the document (creating a brand-new PDF) and it can email that newly created PDF to anyone you choose. Send it to yourself. Once you receive this entirely new PDF, you can publish it on your website, because its metadata has been cleansed of the original traces. Advanced printers — with scanning functions and wireless internet connections — used to be extremely expensive. Today, they are affordable.