Converting a file changes its format, not its quality: a conversion can keep what the original has, but it cannot add detail that was never there. Knowing that helps you choose the right route.
Documents. Word, Excel and PowerPoint files convert to PDF reliably because a PDF is a fixed picture of the layout. The reverse is harder. PDF to Word has to rebuild paragraphs, tables and columns from a page that only stores where each piece of text sits, so complex layouts may need tidying afterwards. PDF to Excel works best on PDFs that contain real tables rather than screenshots of tables. A scanned PDF holds pictures of text, so converting it to an editable format needs text recognition, which plain conversion does not provide.
Images. JPG is made for photographs and keeps files small by discarding some detail. PNG keeps every pixel and supports transparency, which suits logos, screenshots and diagrams, but the files are larger. JPG to PNG is useful when a form or design tool needs PNG, but it will not restore detail already lost when the JPG was saved. Images to PDF is the quick way to send several photos or scans as one document.
Video and audio. The Video & Audio Converter runs inside your browser. That means your files are not uploaded, but a long or high-resolution video uses your device's memory and can take a while, so close other tabs for big files. MP4 plays almost everywhere; pulling the sound out as MP3 is the usual choice for lectures and voice notes.
Units. The Unit Converter covers everyday measurements such as kilograms to pounds, kilometres to miles and Celsius to Fahrenheit.
Privacy. As the note above says, document conversions are handled by iLovePDF's servers and deleted after conversion, while video and audio stay on your device. For a contract, ID or medical record, think about whether you are comfortable uploading it at all, and remove sensitive pages first with Split PDF if you only need part of the file.