Acer Iconia W510, as many other Windows 8 devices, comes with UEFI Secure Boot enabled in BIOS, which prevents booting from unauthorized devices.
Acer didn’t make disabling Secure Boot in W510 very straight forward.

This is what you need to do:

• Shut down the computer.
• Enter BIOS setup (Power + Vol Down)
• Enable BIOS password: Security > Supervisor Password
  Don’t forget this password. There is no easy way to reset it!
• Erase secure boot settings: Security > Erase all Secure Boot Settings
• Disable secure boot: Boot > Enable or Disable secure boot check
• Change Boot Order to boot from USB first: Boot > Boot Priority Order
• Enable boot menu (optional): Main > F12 Boot Menu
• Save Changes: Exit > Exit Saving Changes

Even after this, I had no luck in booting from Windows 8.1 bootable flash drive formatted in NTFS. After reformatting in FAT32, everything worked fine.

Check here to find few useful tools to create bootable flash drives, including Universal USB Installer, which allows to choose FAT32 file system.