You mean view them in something like Explorer? (you didn't mention OS, so that will matter).
If you aren't trying to actually edit the photos, you have a few options.
1) Find a codec for PEF files (google is a friend on this one). Be sure to reference your OS (include version and be careful of whether your OS is 32 bit or 64 bit).
* Since you noted OS, I would note Microsoft used to have codecs you could download, and Ricoh has some. Just watch for 32 vs 64. FastPictureViewer has a great codec pack, but it does cost $10~$20.
2) Alternatively, find a software for quickly browsing photos (and more). These will work in a similar manner as Windows Explorer but geared towards photos and they won't need a codec pack. The advantage is they will allow you to browse by thumbnails and view as a slideshow within their programs. In terms of free/open source type software I might suggest the following.
XnView MP (I like this one these days as a quick viewer, and I use it to cull photos after offloading from my cards)
IrfanView (I used to use this one but haven't in a while, but not because it was bad or anything)
FastStone Image Viewer (I've used this the most, and only recently have I liked XnView a bit more)
These are not the only ones out there, paid and free. I just like them because they are light and I've used them all at one time or other over the past 15 years. I think you can use Adobe Bridge for free, and it is arguably better than the ones above, but I've quit using Adobe products, and it is a bit more bloated than the above.
Last edited by emalvick; 07-20-2018 at 08:48 AM.
Reason: added notes regarding OS