Best graphic file for when printing posters is PDF. Also remember to make your file in CMYK and set the resolution to 300 dpi.
Which graphics file format is best to use when?
JPEG: is best for photos when you need to keep the file size small. JPEG is also lossy so over time the quality of the photo will decrease.
TIFF: is best used for any type of bitmap images used for print because it used CMYK color. It produces large files due to is usual resolution of 3oo ppi and it is not lossy. It is also able to keep layers, transparency, and any other features when saved from Photoshop.
PSD: is used when you want to keep layers, transparency, adjustment laters, masks, clipping masks, blend modes, vector text, etc. Just remember only Photoshop can open this file.
BMP: Huge files but not lossy. Has same benefits as TIFF. It is rarely used today.
PNG: is used when you need small file sizes with no loss in quality. Supports transparency and was made to replace GIF.
GIF: is used for simple web graphics. Only uses 256 colors which keeps file size down and load quick on the web. Can be used for small web animations.