A PDF is supposed to be a fixed, portable format, same document, same appearance, anywhere, which is literally what the name stands for. In practice, the same file can look slightly (or significantly) different opened in Adobe Acrobat, a browser's built-in PDF viewer, and a phone's default reader, different line breaks, different font weight, sometimes different colors entirely. That inconsistency comes down to a few specific, well-understood causes, not randomness.
Unembedded fonts are the most common cause
A PDF can either embed the actual font data it uses directly inside the file, or just reference a font by name and expect the viewing system to supply it. If a font isn't embedded and the viewing device doesn't have that exact font installed, the viewer silently substitutes a similar-looking one, which shifts character widths, line breaks, and sometimes overall page layout, especially in text-dense documents. This is why a document that looks perfect on the machine that created it can look subtly wrong on someone else's device: the creating machine had the font installed, the recipient's didn't.
The fix is embedding fonts at creation time, most PDF export settings have an "embed all fonts" option, and it's worth checking that setting is enabled specifically for any document being widely shared, since it removes this entire category of inconsistency.
Rendering engines interpret some things differently
Beyond fonts, different PDF rendering engines (the software underneath each viewer) can handle color profiles, transparency effects, and anti-aliasing slightly differently, which produces small visual differences even when the underlying file is byte-for-byte identical. This is more noticeable in design-heavy documents with gradients, transparency, or specific color profiles (CMYK vs RGB) than in plain text documents, where there's less room for interpretation to diverge.
How to check for a consistent result
Before sending an important document widely, opening it in more than one viewer, ideally a desktop PDF reader and a browser-based one, catches most rendering surprises before a recipient does. If a document's appearance genuinely needs to be locked down regardless of viewer or available fonts, converting text-heavy layout elements to outlines or flattening the visual appearance with Flatten PDF removes the dependency on font substitution and interactive rendering choices entirely, since flattened content is drawn once and doesn't get reinterpreted per viewer.