As the years have gone by, Adobe Reader (formally Adobe Acrobat Reader) has got bigger (version 5.05 is 8.6MB and will probably still handle many PDF's OK, the latest, 8.1.1 is 22.3MB) and invariably slower with each release. Thankfully there are free alternatives, that are both a smaller download and much faster at opening PDF's.
- SumatraPDF (< 1MB, still in beta, open source)
- Foxit Reader (around 2MB download)
- PDF XChange Viewer (13MB, but still smaller than Adobe Reader and supports comments/annotations, filling in forms and exporting text and images)
Alternatively, you could still have Adobe Reader, and use a tool that disables the plugins that slow the startup - e.g. Adobe Reader SpeedUp or PDF SpeedUp (although I haven't tested either of these).