Why do you prefer PDF resumes and not .Doc? What's wrong with Photoshop resumes?

You should always use PDFs when submitting your resume online, unless the Word document is explicitly asked for. 

Important: Your PDF must not be a scanned image

That said, the PDF should always contain text and not be a scanned document/image. A simple way to test this is by opening your PDF in something like Chrome, Adobe Acrobat Reader or Preview, and checking if you can highlight all the text. If you can, your resume is not a scanned image and it should be read correctly. If you can't, then your resume is likely in an image format and won't be read by resume screeners.

This includes resumes designed in tools like Photoshop and other design tools, which should be avoided if you are sending your resume online. Unfortunately, scanned images do not get read correctly by Applicant Tracking Systems - AKA automated resume screening software - which could mean your resume will never see the eyes of a recruiter.

The easiest way to be sure if your resume PDF will be created correctly is to generate your resume PDFs via either Word or Google Docs (Free). 

Why you should stick to PDF:

• Preserved layout: Word documents and their layouts appear differently on different versions of Word or operating systems. For example, your resume that looks one page on your computer may look like 2 pages on another computer. You want to make sure your resume appears correctly on all machines.
• Many Applicant Tracking Systems cannot read Word documents correctly. Some software don't accept Word altogether, while many incorrectly process Word files and don't read the fonts correctly if they aren't common fonts. 
• A PDF cannot be edited easily like a Word document. Leaving your resume in Word exposes you to the possibility that someone might inadvertently (or intentionally) alter it.
• If you are using a font an employer does not have installed and you send a Word file, your resume won't appear correctly - instead, it would appear as special (unreadable, think Wingdings) characters on their computers. 
• Finally, not everyone has Office installed - for example, a recruiter or hiring manager could be using a computer that hasn't been completely configured yet. If they don't have Office, they're not going to be able to read your resume. On the other hand, PDFs can be opened directly via email or the browser making them viewable on every platform.