Guides
Reference pages, written to be cited
Short answers with the numbers behind them. Where a figure comes from a measurement, the page says what was measured and when.
- ImagesHEIC to JPG on iPhone, Mac and WindowsTurn iPhone HEIC photos into JPG for free on iPhone and Mac, or with Paint on Windows after two Store extensions, one of them 0.99 US dollars in the US store.
- PrivacyHow long free PDF websites keep your uploaded filesWhere eight free PDF websites process a file and how long each keeps it, in the companies' own words, with links, checked on 26 September 2026.
- Page sizesA4, Letter, Legal, A3 and A5 in pixelsExact pixel sizes of A4, Letter and three more pages at 72 to 600 dpi, how the numbers are worked out, and which setting to pick for screen, print and scans.
- MeasuredJPG, PNG or WebP: measured file sizesThe same three images saved in each format at quality 80 by this site's converter, with the file sizes, and a plain rule for choosing between them.
- MeasuredPDF compression, measured on a scan and a text fileMeasured results of compressing a 14.86 MB scan and a 21 KB text PDF, why the text file cannot shrink, and how to choose a compression setting.
- LicensingPDF library licences: which allow commercial useEighteen widely used PDF libraries and tools with their licence and a plain yes or no on using them in a paid or ad-funded product without buying a licence.
- PDF, plainlyWhat a browser can and cannot do to a PDFThe exact list of PDF jobs a modern browser handles on its own, the jobs it cannot, and the memory arithmetic that sets the file-size limit.
- SpreadsheetsWhy Excel removes leading zeros and changes CSV dataThe seven changes Excel silently makes when it opens a CSV file, with examples, and the import method that prevents every one of them.