New all in one printer/scanner. when I scan I get:- Adobe reader could not open 'scan_Doc0001.pdf'

Might need a bit more information about this: But I'm presuming you scan your document and save it as a pdf document using the built in HP software?
Couple of things to try:
1 - have you got the latest version of Adobe Reader? It may be that you are using an older version - the pdf document specification has been updated over the years and it may be that you're saving the pdf using a newer version of the pdf format than adobe reader is able to read.
2 - Adobe reader isn't the only pdf viewer, there are numerous other pdf document viewers (some free some not) You could try another pdf viewer, like Foxit Reader (free to use) and see if that can open it?
3 - When you save the scan, make sure that in the Save Dialog box it is really saving it as pdf. Common mistake is that people often just save a file with .pdf and expect the program to know that you want to save as pdf. When you scan there are several other file formats that you can use (Tiff, Jpeg etc) Is is possible you're saving the file as Tiff or Jpeg but with a .pdf file extension? If you're not sure, re-scan the document and resave it, but don't use a file extension, let the scanning program do that, see what it saves it as?
4 - check where you're saving the pdf on your computer? Is the place you're trying to open it from, the same as the place you've saved it?

Hi I recently bought a cannon pixma MG3250. It is scanning photos okay but when I go to scan a document it scans it in PDF form which I can open on my pc but when I save it I cant access it. Can you help me? Lucy

Scan the document with HP tool box