There are two types of downloads available in ProQuest Ebook Central:

Download PDF Chapter 
Permits users to download ebook chapters in PDF format, there is a daily limit on how many pages can be downloaded/printed.

Download Book
Downloads an ACSM file, this option may be disabled if the book has a simultaneous user limit and is already in use, or, if the item is on Course Reserve.

Why did an ACSM file download to my computer instead of a PDF?
ACSM stands for Adobe Content Server Message. ACSM files communicate with the Adobe server to access PDFs that are protected by Digital Rights Management (DRM). To open ACSM files you will need to install Adobe Digital Editions.  ProQuest Ebook Central will automatically walk you through installing the software if you download an ebook and select 'I am using my own computer'.

If you would like to access the ebook across all of your devices, you can create a free Adobe ID account. To associate Adobe Digital Editions with your Adobe ID select Help from the program menu, click Authorize Computer and follow the prompts to log in.

Adobe Digital Editions can be used without an account, but you will be restricted to reading ebooks on the one device you downloaded them to. 

Why are ebooks not displaying correctly in Adobe Digital Editions on iOS?
ProQuest has suggested a workaround for ebooks that aren’t displaying correctly in Adobe Digital Editions on iOS: Illegible text and missing pages will be fixed if you change the display settings to a sepia or black background.

ProQuest has created a more detailed troubleshooting guide here.