Troubleshooting methods:
Read any PDF file on an iPhone. On the bottom menu bar, select Files. Locate the file on your iPhone and select it. You can now scroll through and read your PDF.
The Mail, Safari and iBooks apps preinstalled on the iPhone all allow you to view PDFs on your iPhone. The iBooks app offers the most dynamic set of controls for viewing PDFs, while the PDF viewer associated with Mail and Safari only lets you view PDF documents.
PDF files open natively in the Safari app. Tapping a link to a PDF file will display the PDF file in the browser.
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Opening Email Attachments. Open the email that contains your PDF attachment. Open the message so that you can see the attachment link at the bottom of the screen. Tap the PDF attachment to view it.
Right-click the PDF, choose Open With > Choose default program (or Choose another app in Windows 10). Choose Adobe Acrobat Reader DC or Adobe Acrobat DC in the list of programs, and then do one of the following: (Windows 7 and earlier) Select Always use the selected program to open this kind of file.
Go to File > Open. Find the PDF and open it (you might have to select Browse and find the PDF in a folder). Word tells you that it's going to make a copy of the PDF and convert its contents into a format that Word can display. The original PDF won't be changed at all.
To open PDFs using an iPad or iPhone in Safari follow the below steps:
From the Finder, Control-click the file, then choose Open With > Keynote. If Keynote is the only presentation app on your Mac, you can just double-click the file. From the Keynote for Mac app, choose File > Open, select the file, then click Open.