Every file format is a set of rules for storing data - text, pixels, audio samples, frames - in a particular layout. Converting a file means reading it under one set of rules and re-writing it under another. That sounds simple, and for some formats it is. For others, it's the difference between a perfect copy and a noticeably worse one.
What Actually Happens During Conversion
A converter does three things: parse the source file's structure, map its content onto the target format's structure, and write the result. The part that matters most is the mapping step, because not every format can represent the same things.
- Lossless conversions (like PNG to JPEG being the exception aside - most image format swaps where structure maps 1:1, or DOCX to PDF) preserve the original data exactly, or close to it.
- Lossy conversions (JPEG re-compression, MP3 bitrate changes, most video re-encodes) throw away some data permanently to save space. Do it once and you usually won't notice. Do it five times in a row and you will.
This is why "convert PDF to Word and back to PDF" sometimes mangles formatting - DOCX and PDF don't represent layout the same way, so the round-trip isn't lossless even though no one intended to lose anything.
Document Conversion: Where Formatting Actually Breaks
PDF is a fixed-layout format - every character and image has an exact x/y position. Word (DOCX) is a flow-layout format - text reflows based on margins, fonts, and page size. Converting PDF to Word means guessing where paragraph breaks, tables, and columns should go, because that information isn't stored the same way in PDF.
What this means practically:
- A simple, single-column PDF (a letter, an invoice, a contract) converts to editable Word almost perfectly.
- A PDF with multi-column layouts, embedded charts, or scanned (non-text) pages converts less cleanly - scanned pages need OCR (optical character recognition) to even extract text at all, and OCR accuracy drops with low-resolution scans or unusual fonts.
- Word to PDF is the easier direction - DOCX's flow layout maps onto PDF's fixed layout without ambiguity, so this conversion is reliably clean.
The same asymmetry shows up with ODT to DOCX: both are flow-layout formats, so most documents convert with no visible difference, but OpenDocument-specific features (some table styles, certain macros) don't have a DOCX equivalent and get dropped.
Image Conversion: Compression, Not Just Format
Image formats split into two groups: ones that compress lossily (JPEG) and ones that don't, or compress losslessly (PNG, WebP in lossless mode, TIFF). Knowing which group you're in/out of changes what you should expect:
- HEIC to JPG - both are lossy, but HEIC's compression (based on the HEVC codec) is more efficient than JPEG's. Converting HEIC→JPG at high quality settings loses very little, which is why it's the default fix for "my iPhone photos won't open on Windows."
- JPG to PNG - going lossy→lossless doesn't recover any quality the JPEG already lost; it just stops further loss and adds transparency support.
- TIFF to GIF - TIFF supports millions of colors, GIF supports 256. This conversion will visibly reduce color quality on photos; it's fine for simple graphics and icons.
- Repeated JPEG re-saves compound quality loss. If you need to edit a JPEG multiple times, convert to PNG first, edit, then export to JPEG once at the end.
Audio and Video: Codec vs Container
This is the most commonly misunderstood part of conversion. A video file like .mp4 or .mkv is a container - it holds separate video and audio streams, each encoded with its own codec (H.264, H.265, AAC, etc.). Converting "MOV to MP4" usually just repackages the existing streams into a new container, which is fast and lossless. Converting between codecs (re-encoding H.264 to H.265) is slower and is where quality and file size actually change.
- MOV to MP4: typically a container swap, not a re-encode - quick and lossless if the codec inside is already MP4-compatible (it usually is).
- AVI to WMV: AVI is an older, less efficient container; this conversion does usually involve re-encoding, so expect a real (if small) quality trade-off.
- MP3 to WAV: going lossy → lossless (uncompressed) doesn't restore audio detail the MP3 already discarded - the WAV file will be larger but not "better" than the source MP3 was.
- AAC and OGG (via AAC and OGG conversion) are both lossy codecs with similar quality at the same bitrate; converting between them at a high bitrate (192kbps+) is usually inaudible, but converting down in bitrate is not.
Picking the Right Format
- Send a document nobody should edit → PDF
- Share a document people need to edit → DOCX
- Photo for the web → JPEG (photos) or WebP (smaller, supports transparency)
- Screenshot, logo, or UI graphic → PNG
- Archive video at max quality → MKV with H.265
- Video for universal playback → MP4 with H.264
- Audio for editing/production → WAV or FLAC (lossless)
- Audio for everyday listening → MP3 or AAC at 192–320kbps
Common Conversion Problems
- A scanned PDF won't convert to editable text. That's an OCR problem, not a conversion problem - the "text" in a scanned PDF is actually a picture of text. Make sure OCR is enabled for scanned documents specifically.
- A converted video plays with no sound, or vice versa. Usually means the target container doesn't support the source's audio codec. Re-encoding the audio track (not just repackaging) fixes this.
- Repeated conversions look progressively worse. Expected for lossy formats. Keep an original master file and convert from it each time, rather than converting a conversion.
- File is rejected for being "too large." Most online converters cap upload size; for video specifically, re-encoding to a more efficient codec (H.265 instead of H.264) can cut file size 30–50% at the same visual quality.
Browse All Conversions
RunConvert supports conversion across documents, images, audio, video, archives, fonts, vector graphics, and CAD formats - all processed in the browser, with files deleted automatically after conversion.